How old was Bhisma in Mahabharata?

First we need to figure out how old Bhisma was during his meeting with Santanu. http://www.sacred-texts.com/hin/m01/m01101.htm

The Mahabharata states that Santanu enjoyed married life for 36 years then retired to the woods, over their he had another meeting with Ganga where Bhisma was introduced:

And during the rule of the best of Kurus--of that king of kings--speech became united with truth, and the minds of men were directed towards liberality and virtue. And Santanu, having enjoyed domestic felicity for six and thirty years, retired into the woods.

• If you subtract the eight years (of giving birth to others sons of santanu that died) then the number is 28 instead of 36.
• Santanu had 8 years of marriage with Ganga
• Santanu had another 28 years of marriage with Satyavati
• Bhishma's introduction http://www.sacred-texts.com/hin/m01/m01101.htm

And while seeking for a cause, the illustrious monarch beheld that a youth of great comeliness, well-built and amiable person, like Indra himself, had, by his keen celestial weapon, checked the flow of the river. And the king, beholding this extraordinary feat of the river Ganga having been checked in her course near where that youth stood, became very much surprised. This youth was no other than Santanu's son himself. But as Santanu had seen his son only once a few moments after his birth, he had not sufficient recollection to identify that infant with the youth before his eyes. The youth, however, seeing his father, knew him at once, but instead of disclosing himself, he clouded the king's perception by his celestial powers of illusion and disappeared in his very sight.

• This is proof that bhisma was a youth (teenager). 
• Youths are in between 15 and 24 years old. 
• So at the least Bhisma was 15 years old, at the most he was 24. 
• Since Santanu would not stay unmarried for 24 years of his adult life 
• I’ll assume that Bhisma was around 17 years old (1+7=8, he’s the 8th son).

• Introduction of Satyavati (4 aka four years later) http://www.sacred-texts.com/hin/m01/m01101.htm

Four years had thus passed away, when the king one day went into the woods on the bank of the Yamuna. And while the king was rambling there, he perceived a sweet scent coming from an unknown direction. And the monarch, impelled by the desire of ascertaining the cause, wandered hither and thither. And in course of his ramble, he beheld a black-eyed maiden of celestial beauty, the daughter of a fisherman. The king addressing her, said, 'Who art thou, and whose daughter?

• Bhisma is now 21 years old.
• The text literally says 4 years.
• 28 more years passed by (refer to the statement of 36 years of marital life in the first quote):
• http://www.sacred-texts.com/hin/m01/m01102.htm

'O monarch, after the nuptials were over, king Santanu established his beautiful bride in his household. Soon after was born of Satyavati an intelligent and heroic son of Santanu named Chitrangada. He was endued with great energy and became an eminent man. The lord Santanu of great prowess also begat upon Satyavati another son named Vichitravirya, who became a mighty bowman and who became king after his father. And before that bull among men, viz., Vichitravirya, attained to majority, the wise king Santanu realised the inevitable influence of Time. And after Santanu had ascended to heaven. Bhishma, placing himself under the command of Satyavati, installed that suppressor of foes, viz., Chitrangada, on the throne, who, having soon vanquished by his prowess all monarchs, considered not any man as his equal.

• Santanu dies and Chitrangada is coronated
• Chitrangada’s age is irrelevant we all know he was older than 20 year of age of course.
• By now Bhisma is 49 years old (28 plus 21)
• Three more years pass by: http://www.sacred-texts.com/hin/m01/m01102.htm

Between that Gandharva and that foremost one of the Kurus, who were both very powerful, there occurred on the field of Kurukshetra a fierce combat which lasted full three years on the banks of the Saraswati. In that terrible encounter characterised by thick showers of weapons and in which the combatants ground each other fiercely, the Gandharva, who had greater prowess or strategic deception, slew the Kuru prince.

• Chitrangada dies after 3 years.
• Now Bhisma is 52 years old.
• Vichitravirya is crowned.
• Vichitravirya becomes of marriageable age: http://www.sacred-texts.com/hin/m01/m01103.htm

'O thou of Kuru's race, after Chitrangada was slain, his successor Vichitravirya being a minor, Bhishma ruled the kingdom, placing himself under the command of Satyavati. When he saw that his brother, who was the foremost of intelligent men, attained to majority, Bhishma set his heart upon marrying Vichitravirya.

• He has to be at-least 15 to get married. 
• I will say 2 years passed by after the death of Chitrangada turning Vichtravirya into a youth. 
• Bhisma is now 54 years old.
• Death of Vichitravirya: http://www.sacred-texts.com/hin/m01/m01103.htm

And Vichitravirya also, endued with the prowess of the celestials and the beauty of the twin Aswins, could steal the heart of any beautiful woman. And the prince passed seven years uninterruptedly in the company of his wives. He was attacked while yet in the prime of youth, with phthisis. 

• Prime of youth (at the peak of his young days).
• This implies he was the oldest a youth could be (24) so he was 24 when he died. But 15 plus 7 equals 22. 
• This makes Bhisma (he was 54 before) 61 years old.
• Birth of Pandu: http://www.sacred-texts.com/hin/m01/m01107.htm

Ambalika, then, when her time came, brought forth a son of pale complexion. Blazing with beauty the child was endued with all auspicious marks. Indeed, it was this child who afterwards became the father of those mighty archers, the Pandavas. 

• A year after Vichitravirya’s death.
• So Bhisma is 62 years old now.
• Pandu’s coronation: http://www.sacred-texts.com/hin/m01/m01110.htm

And they grew up into fine young men skilled in the Vedas and all athletic sports. And they became well-skilled in the practice of bow, in horsemanship, in encounters with mace, sword and shield, in the management of elephants in battle, and in the science of morality. Well-read in history and the Puranas and various branches of learning, and acquainted with the truths of the Vedas and their branches they acquired knowledge, which was versatile and deep. And Pandu, possessed of great prowess, excelled all men in archery while Dhritarashtra excelled all in personal strength, while in the three worlds there was no one equal to Vidura in devotion to virtue and in the knowledge of the dictates of morality. And beholding the restoration of the extinct line of Santanu, the saying became current in all countries that among mothers of heroes, the daughters of the king of Kasi were the first; that among countries Kurujangala was the first; that among virtuous men, Vidura was the first; that among cities Hastinapura was the first. Pandu became king, for Dhritarashtra, owing to the blindness, and Vidura, for his birth by a Sudra woman, did not obtain the kingdom. 

• How old would Pandu be when he is king? 
• Pandu would be at the very least a minor (like Vichitravirya). 
• I think Pandu was 23 years old, because the text calls him a young man.
• Making Bhisma 85 years old.
• Birth of Yudhishthira 
• A year for the digvijaya of Pandu.
• Gandhari was pregnant for a year.
• During this time the curse of Kindama would happen.
• Thus two years passed by making Bhisma 87 years old.
• Death of Pandu http://www.sacred-texts.com/hin/m01/m01127.htm

The birth, growth, and Vedic studies of these children of Pandu, will, no doubt, give you great pleasure. Steadily adhering to the path of the virtuous and the wise, and leaving behind him these children, Pandu departed hence seventeen days ago.

• Sahadeva (the youngest son of Pandu) had already studied the vedas by now. And he was born 4 years after Yudhishthira. 
• You would have to be at the very least 5 years old to study something. Sahadeva was the most wise among all the brothers so 5 years of age seems like a good estimate.
• 9 years passed by (Yudhishthira was four years older than Sahadeva). 
• Bhisma is now 96 years old.
• Swayamvara of Draupadi 
• Arjuna was 7 years old when Pandu died. 
• 26 - 19 = 7.
• He was 26 years old during the Swayamvara.
• This happened 19 years after the death of Pandu 
• Arjuna was 2 years older than Sahadeva. 
• Same Arjuna was 18 during the Tournament of Adi Parva.
• Arjuna was 22 when the house of lac burned down
• He was 24 when he left Ghatotkaca.
• He was 25 when he disguised himself as a rishi to beg for alms.
•  Arjuna was 26 during the Swayamvara.
• Bhisma would be 115 years old now.
• Arjuna-Vanavasa
• Pandavas spend a year building Indraprastha.
• They spend another year with their wife Droupadi and conquering kingdoms.
• Arjuna gets exiled for twelve years, thus combined they are fourteen years altogether.
• You do the math 14 plus (+) 115 equals (=) 129 years.
• Khandava- Daha
• The youngest son of Arjuna would be born after the exile. 
• He would be fourteen years old when he starts to partake under military training by Arjuna.
• Another year to complete his training.
• A year for him to be born so 16 years passed by.
• Now Bhisma is 145 years old.
• Udyoga Parva (right before his death in the great war)
• Dhritarashtra’s statement of 33 years http://www.sacred-texts.com/hin/m05/m05052.htm

Three and thirty years have passed away since the time, when Arjuna, having invited Agni, gratified him at Khandava, vanquishing all the celestials. 

• I decided to go with Dhritarashtra's statement this time instead of Arjuna's. 
• Because it makes a lot more sense for 33 years to pass by instead of 65 years.
• Anyways this would make Bhisma 178 years old, during his death in the Kurukshetra war.
• I know 178 is hard to believe because it is impossible for any human being to fight so well at such a age. But their are 100 year old bodybuilders in india. 
• Proof https://www.mensxp.com/health/body-building/26774-the-father-of-indian-bodybuilding-manhor-aich-is-102-years-old-but-still-far-from-giving-up.html manhor aich is close to the age of Bhisma.