He doesn't look 21 minimum in the second picture...Maybe a bit more beard and a bit more facial creasing on the forehead and around the eyes.

^^ agreed. The older version of the boy has the same neck. you gave him more bicep and shoulder, but he needs more neck, he looks like he is trying on his Dad's leather jacket.

still looks the same age, just with hair and a shirt.

His face is still round (children have round faces comparing to adults); I suggest you adding Adam's apple too, besides a thicker neck.

I know it's subtle, but I can see the changes are very well done... (the high res is awesome) the work is very good as well..... but as the comments you've gotten tell me you may have to add a little more..

good luck author.. it's a very good chop.. just age issues