Why were the Triarii differently equipped?

[Walhaz]

Another argument for the aggressive usefulness of the triarii is the simple fact that they were included in the legion at all. Rome was capable of calling on vast reserves of manpower. During the 2nd Punic War Hannibal wiped out something like 16 legions in less than three years, but the Romans had raised just as many and more even before the shock of Cannae wore off. I seriously doubt that the pragmatic Romans would bother with calling up 600 decrepit old reservists to guard the legion camp when they had the manpower reserves to call up 600 able-bodied 18-20 year olds who could accomplish the same task. Basically what I'm saying is that if there were any benefit to an army made up entirely of 18-20 year olds, then the Romans could very well have fielded armies like that, but they didn't. And if they included 35-46 year olds, then those 35-46 year olds could do the same things as the younger soldiers because if they couldn't, well then there just wasn't any point to bringing them at all and they would have been replaced with younger soldiers.