I don't think you can guess age from length very well.

I had an old male that was about 4.25". He was WC and showed some wear on his shell.

I had a CB male who reached 4" by age 2 and about 4.5" at age 3.

The diet looks OK. Are you adding any calcium/d3 powder? If he's indoors all or most of the time, he'll need some d3 supplement. I use Reptivite calcium w/d3 powder. A little sprinkle every other meal.