Logically there's no way Tobias and Ax's human morphs aged. If morphs aged then their house fly morphs would've died of old age long before the series ended.
As for morphing a creature at the age it was when acquired, I believe that's something to do with telomeres, which shorten as you age. That's how you can tell roughly how old someone is from their DNA.
And as interesting as this discussion is, it's not the right topic for it. On a topic related note, which humans do you have access too? Humans are animals too after all. So which famous, important or powerful people might you be able to acquire if you had the morphing power?