A creatu turns into a teenager at 15 days and into an adult at 30 days. However, they only age on your profile, not in a rancher shop or a showroom. If you are a rancher, you cannot sell the eggs, that is reserved for merchants as eggs are technically "items". You can only sell the creatu as a rancher.

To change your avatar, go to Preferences. It can be found in the navigation bar on the left or in the row of icons at the top of the page (it's the first one, looks kinda like a spoon/screwdriver). Once there, go to Forum Options (can't miss it). Then wham, you're in the place to change your avatar. I hope that helped.

Also, this is the "Suggestions and Ideas" forum, next time you have a question, post in the Support forum please.