Why were the Triarii differently equipped?

[JaM]

that might be the case in modern times, but ancient times are different. Humans aged faster. 20 years old men was at top of his physical strength, he had best stamina he could have. once you are older and older, you are going down, and amount of training is just making difference smaller, but it doesn't disappear, even today.. Hastati were used as first line of attack because they were young, therefore could tire down enemy without being tired as fast themselves. Once they were withdrawn, Principes attacked already tired enemy and would decide the fight (most likely locally, not on the whole line)