There´s no way of knowing it´s age.
Ian´s stimate is fairly good, but it´s an stimate nevertheless....it could be 5 months old, or 3 years old.
I´ve had metamorphs that reached 4,5 cm long and over a year old juveniles that didn´t grow bigger than that, so it´s all extremely relative.
As for foods, earthworms are indeed the very best. Small slugs, isopods, gammarids, daphnia, waxworms, etc, are also excelent food items, and are extremely cheap if you culture them