Yeah I agree, I feel like as a child it wouldn't have been a bad thing to do. It would have been cool if they got their tails to move around, but couldn't use any of the powers until they were a teen. Then it would feel a bit more like a progression, and as @annaliese39 said, ghost and aliens are kids... why not just have tails as a kid?
Mermaids feel very patched in to me anyway.. there's no skill or progression for them.. and it's way too easy to become one in the first place. It just feels odd and empty, very disappointing from the sims team because TS4 has been about developing everything to feel grounded in the game.
Also, I agree with vampires being a 'it kicks in as a teen'. I'm completely okay with that, I just bet that witches/warlocks will have powers a child too.