Fit and flare is a classic shape! Absolutely never too young. It isn't the style that makes them too young. I'd read Anna's original post about the dress and I think that I can explain why that particular dress got the "too young, thumbs down." It is a fitted jersey with a high waist, short skirt and racer back. All of those things are difficult.
Cotton jersey reads as very casual. Some synthetic jersey is very hard to wear too, showing all lumps and bumps and even creating some you don't even have. (Anna got a thumbs down on an Acne jersey dress a while back too.) Neither has a lot of structure to them. The same style dress in cotton woven or wool would look entirely different. That extra structure will make it less casual, more grown up.
The high waist in and of itself is not a problem. Paired with a short skirt, though, it reads very young. A longer skirt would have been entirely different looking. You'll notice that a few people wondered about the length in the comments.
Racer back. Well, that's pretty straight forward. A racer back style is very casual, no matter what the length of the dress or the fabric. If you are going for a very casual look, it is okay and probably wouldn't kick something into "too young," but combine it with the short length and cotton jersey and it is too young looking.
So get out there and buy yourself some fit and flare dresses! They are a natural for your shape and you are going to look gorgeous and totally age appropriate in them.
