Characters are
guaranteed three pieces of art if they RP unless otherwise stated in contests. However, these characters (Unless you're getting a Dibbun which is explained later!) do not have stages. That's correct - Characters do not have stages.
After certain periods of time or major changes take place, a character will get new art (you MAY request when this happens, but you are not guaranteed an approval!). Or the artist could randomly redraw active characters to reflect RP. If you RP a lot, chances are, artists are going to notice you, think "HOLY GOD THATS COOL" and draw you another piece of art if they can. You can get more than three pieces of art if you RP. Again: RPing is conductive to getting art.
If you don't RP, the chances of you getting new art is pretty much nil so you WILL only get one piece. So if you don't plan on RPing your character, you're basically going to get one piece of art as a pretty sig pet.
Dibbuns
Dibbuns are THE exception to the rule. Dibbuns will start as toddlers (unless they're 2nd Gen XD That's described later too), grow to child, and then onto adult. These are done at artist's discretion though, as above, you can make REQUESTS. These requests can be denied, though.
Second Gen Dibbuns
These are the only characters that are guaranteed FOUR pieces of art. Infant (Which is the shortest), Toddler, Child and Adult.
Second Gen can be kept as secondary characters for the couple breeding or can be given away to newcomers. It is preferred to be given away to newcomers, though.
The stipulation with giving it away, however, is the new comer will be part of a family RP and will be largely influenced by this. For example: You want your little mousebabe to grow up and be evil and join the vermin!
Great. Only problem is your mousebabe's family is sweet as pie and always showers with love. Your little mousebabe going evil? Sorry, not going to happen.
Thus, you'll have to get another character for your plotpoint of going evil and running away with the vermin to come to be and make sense.