Do we know if this is a boar or sow? I know you are curious about the cavy's age, but it really doesn't matter very much insofar as what to feed it. If the diet consists of good quality pellets, hay, and some veggies, s/he will do just fine. I have two very young boars and I make no effort to feed them anything special. The only way that I take age into consideration is that I'm less cautious about feeding high calcium veggies until they are about 6 mos. old. After that age I'm quite careful to restrict the calcium intake for boars.
I'm sure that you know this, but a sow's urethra is short and wider and a stone may be less problematic for her. OTOH, a boar's urethra is long and narrow and they rarely can pass stones. As an example, a few years ago I had a sow who had a stone lodge in the wall of her urethra.......and stayed there until her death. Other than a rare UTI, it didn't seem to cause any problems.