There have been many times over the years when I have heard, from many different people, the statement of "I wish I was still as creative as I was when I was younger."
You know, when I was younger, and I mean... -really- young... I used to just string together a bunch of letters, then show them to my parents and ask them what they words they were. And the answer I always got back was "Nothing. ...It's just gibberish."
Now I'm older and I understand how sentence structure works, and word definitions, and the proper formats and ways to use text in order to get complex and intricate ideas out to other people.
I don't think we became 'less creative'. I think we simply understand now how things work. Unless you're trying to define 'creative' as 'uneducated'. When you're little, you're not really certain how a goat-creature is going to have seven heads, three pairs of wings, and six sets of legs. You just draw it because it looks cool. But as you get older you get a sense of anatomy and attempt to work things out so that someone else could look at them and go "Yeah. You know what? That seems reasonable and believable." Maybe it doesn't have seven heads and twelve legs anymore, but you know what? That doesn't make you less creative.
It just makes you more thoughtful.
And if anything, it makes you -more- creative because you're considering in-depth details that you would have otherwise overlooked. We're treading on a dangerous ground here where 'creative' almost implies 'ignorant to details', and that really shouldn't be.
If you WANT to make a seven-headed goat-creature with twelve legs and six wings, then you have every right to, and just because you know better doesn't mean it should stop you. But don't consider yourself less creative simply because you understand more about how anatomy and structure works, and because your brain happens to take that into consideration possibly without you even realizing it at first.