How old was Devavrata aka Bhishma?

If you are an Indian or are familiar with Indian culture, you must be aware of the Great Indian Epic Mahabharata. The story is rich in that it spans several generations of complex and varied characters that do not fit standard patterns of good and evil.

One of my favorite character is Devavrata (देवव्रत) also known as Bhishma (भीष्मः). It is well known that although he was addressed respectfully as grandfather, he had sired no children of his own and took care of his half-brother's children and grandchildren, having himself accepted a life of celibacy. 

He was a famous warrior, defeated entire armies single-handedly, never having lost battle, performing as well as men one-fourth his age in the grand war that is either the culmination (or denouement depending on your point of view) of the story of Mahabharata. 

I became curious to see if it was possible to measure his age based on information available in the story itself. It is obvious that the method used is by no means scientific. If you disagree, feel free to leave a note.

Devavrata's father Shantanu must have been in his 20s when he met Ganga. It did not look like Shantanu was married or had parents that arranged a marriage for him. He met Ganga during one of his solo hunting trips in the forest. Around 8 years later (there were 7 children that were born before) Devavrata was born which would make Shantanu around 30 years old. Shantanu is separated from Devavrata for around 12 years when he is taught by eminent masters and groomed to be a King fit for the throne of Hastinapuram (हस्तिनापुरम्). At the age of 42, Shantanu is reunited with his son. Shantanu spends several years in the company of his son and then he meets SatyavatI (सत्यवती). At this point Devavrata is around 18 years old and his father around 48. Devavrata having accepted celibacy (long story), becomes Bhishma the Terrible.

Shantanu lives for a few years and has a two sons, namely, Chitrangada and Vichitraveerya. Assuming that he lived for 5-7 years, he would have died at 55 and Bhishma became the steward of Hastinapuram at the age of 25. Chitrangada dies in a battle at a young age. Vichitraveerya marries Ambika and Ambalika and also dies childless. Assuming he died at the age of 20, this would mean Bhishma again becomes the caretaker at the age of 40. 

After 2-3 years, with Vyasa's help the widows have a son each Dhrithrastra and Pandu. Bhishma's age when Pandu is born around 42. 

Pandu rules for a few year; gives us his throne, in his 20s, due to his inability to sire children. Pandu wanders for several years in the forest with his wives where he has 5 sons and dies around the age of 30 when his eldest son, Yudhisthira, was around 10, the youngest twins Nakula and Sahadeva around 6 years old, and Bhishma around 72.

The 5 brothers aka Pandavas, come back to Hastinapuram and learn from Drona the art of war. As part of Gurudakshina, and they wage war on Drupada, Drona's enemy. Since all the Pandavas went to war, we must assume the youngest, twins were at least 16 years of age. This would put Yudhisthira at 20 and Bhishma around 82.

Yudhisthira is declared crown prince, rules Hastinapuram for 1 year, Duryodhana throws a tantrum, get them exiled to Varanavrata, has his henchmen burn the palace, 5 brothers escape to forest, meet Rakshashas, Bheema gets married and has a son Ghatothacha. Let us say this all this took 2 years. This puts Yudhisthira's age at 23 and Bhishma's 85.

The Pandavas marry Draupadi, settle in Indraprasta, rule for a few year, Draupadi lives 1 year with each brother, according to her vow and has 5 sons. Let us say at this point Yudhisthira is 30 and Bhishma 90 years old.

The Pandavas, spend 13 years in the forest, which would mean Yudhisthira was 43 and Bhishma 103 years old at the start of the war. The war lasted only 18 days and Bhishma lived a few weeks more to ensure he died when the sun moved northward (Uttarayana).

So give or take a few years, Bhishma died at the ripe age of 103.