(note: This is not what the text would suggest)
Bhishma -
Likely 20 when he took the vow
40 - when Vichitravirya dies
60 - when Pandu is coronated
80 - when Kuru princes come of age
100 - at the time of Bharata war
In the text, you have Arjuna's long pilgrimage during the Indraprastha years, the long reign in Indraprastha preceding the 13 years of "exile"
So strictly going by the text would give an implausibly high number
That would make Drona, Kripa just a few yrs younger than Bhishma! And more than a generation older than Dhritarashtra
You don't get that sense in popular depictions of these characters