Why were the Triarii differently equipped?

[Nathan Ross]

35 years old man during ancient times would be in much worse physical condition than modern men of the same age.

I wouldn't be so sure. I suspect that a fit and healthy 35-year-old of the 2nd century BC, accustomed to a simple diet, constant exercise and regular arms training since youth would be in much better physical shape than the average 35-year-old today, accustomed to sitting at a computer all day and maybe going 'running' once a week!

average length of life in ancient times was hardly more than 55-60 years or lower.

Life expectancy in the ancient world is hard to estimate, and confused by massive infant and child mortality (something like 50% of children dying before the age of 10), disease and death in childbirth. A man who survived into his 20s, however, could expect to live into his 60s, and overall age demographics do not appear to be that different to the 19th century. (see Karen Cokayne's Experiencing Old Age in Ancient Rome for more about this)

Varro (in Censorinus, de natalis) divided the span of a life into periods: puer (childhood) from 1-15, adolescens from 15-30, iuvenis (youth) from 30-45, senior (maturity) 45-60 and senex (old age) from 60 onwards. 60 was also the age at which a man was no longer liable for jury service.

So it seems that a man of 35-40 was by no means considered old or incapable, and plenty of Romans lived into their 60s and older. Perhaps their fitness wasn't so great at that age, although there will have been exceptions. In a much later era, at Lincoln in 1217, William Marshall led the English army in battle, in person and in full armour - he was aged about 70. He won the battle too...