There is authentic record on Krishna's exit date but not on his birth year. If there is an authentic year, we would be celebrating it by a year number like how we mention the Kali date as 5123rd year as of Mesha this year (2021). Why no such number for Krishna's birth anniversary?
We know we are in 5123rd year of Kali. This is actually the anniversary of Krishna's departure. We do tarpan on that day even now - it's Mesha maasa pirappu.
If he had lived for 125 years, then it means Arjuna outlived him, for he departed after Krishna. He was three months younger than Krishna but left more than seven months after him . If Arjuna was 125 at his death time, Bhima was 126 years then and Yudhishthira 127 years of age. They lived longer than Krishna. Why these are not found mentioned anywhere?
Looking at the available evidence in Harivamsa, Krishna married Rukmini when she was 16 years old. So Krishna must have been older than 16.
Let's assume he was 20.
They got 10 children.
In the meantime Arjuna visited Dwaraka on his 12 yr vanavasa. After that Pandava exile for 13 yrs. After that they lived for 35 years.
20+ x + 12+ y + 13 + 35
= 80 + x+ y
X+ y occured before exile.
So 32 + x+ y was the age they went on exile.
Assuming x + y = 10 , they were 42 when they went on exile.
Actually x + y were very small time with not much info in Mahabharata.
So roughly we can say the war was fought when they all were around 50 years or in their 50s.
Then they lived for 35 years.
So they all have left in their 80s.