Thank you for your question! If the character is playable in a setting built for humanoids, then that character should be acceptable - I've generalized this to having hands, being two-legged and two-armed, and able to fit inside a normal building, playable without needing extensive accommodations in the game mechanics to fit them specifically.

If there's something else specific to the character that you need clarified, please feel free to give me more detail on the character specifics so I can tailor my answer more!

As for the age cutoff, I don't particularly have one. As long as the character is playable (old enough to speak and have a personality/develop CR) it should be fine. However, be aware that a character who is very young, such as ages 5-6 would face significant limitations in gameplay. For example, they would be placed in foster care with a suitable family unless apped with a guardian from canon, and having the character adopted by another player character may take a while. This might limit the freedom the character has to participate in events etc. but if you are alright with working around those difficulties I don't see why you couldn't app a character that young.