You ignored the Polybius quote about the velites, why? Let me post it again:
"...they choose the youngest and poorest to form the velites..."
Velites were youngest and poorest. Not youngest or poorest.
Be careful when referring to Romans under the age of 47 as junior and seniors, those words, in Latin, already have a fixed definition on this subject, 17-46 were junior, older men were senior, exempt from military service and able to vote in their centuries before the iuniores.
In addition, I think it's much more likely that someone capable of serving in the Triari would have more money for fighting kit than someone in their late teens, early twenties. Namely that the younger soldier would likely not be financially independent, while the man in his late thirties or forties likely doesn't have a living father and is paterfamilia of his own family. Also, they have fifteen to twenty years head start earning/saving money from commerce or accumulating cheaply gained arms or armor while campaigning. Whereas Mr. Tyro Hastati is likely going to be wearing hand me down kit.
Lastly, Republican Rome didn't have a middle class, that's a term coined in 1745 to poorly describe the merchant class of medieval and Renaissance era Europe. Also, hastati wouldn't be middle aged as you wrote, that would be mix between Principes ("prime of their life") and the younger of the Triari, assuming life expectancy is 65-70ish (discounting childhood mortality skewing the stats). That is if the age distinctions followed Polybius' rather strict guidelines, which were surely looser in practice.
"...they choose the youngest and poorest to form the velites..."
Velites were youngest and poorest. Not youngest or poorest.
Be careful when referring to Romans under the age of 47 as junior and seniors, those words, in Latin, already have a fixed definition on this subject, 17-46 were junior, older men were senior, exempt from military service and able to vote in their centuries before the iuniores.
In addition, I think it's much more likely that someone capable of serving in the Triari would have more money for fighting kit than someone in their late teens, early twenties. Namely that the younger soldier would likely not be financially independent, while the man in his late thirties or forties likely doesn't have a living father and is paterfamilia of his own family. Also, they have fifteen to twenty years head start earning/saving money from commerce or accumulating cheaply gained arms or armor while campaigning. Whereas Mr. Tyro Hastati is likely going to be wearing hand me down kit.
Lastly, Republican Rome didn't have a middle class, that's a term coined in 1745 to poorly describe the merchant class of medieval and Renaissance era Europe. Also, hastati wouldn't be middle aged as you wrote, that would be mix between Principes ("prime of their life") and the younger of the Triari, assuming life expectancy is 65-70ish (discounting childhood mortality skewing the stats). That is if the age distinctions followed Polybius' rather strict guidelines, which were surely looser in practice.
