One of the most obvious indication of age is the head to body ratio (and to a smaller extent the hand to body and foot to body ratios) The younger a person is, the bigger their head is in relation to their body. Babies are only 4 heads high which gradually grows to 7-8 heads high as adults.

So your morph needs to reflect this ratio correctly, or it will just look like a scaled down person, not a young one.