For me, characters looking older than they actually are is a bigger problem. Sera from Kore wa Zombie desu ka... in the anime, her character design looks like an onee-san. And I think that her seiyuu's voice is a bit too deep and mature-sounding. As a result, I assumed she had a body type of someone in her 20's and was not a serious candidate for romance... Now that I've seen her illustrations and characterization in the light novel, I realize she's a cute and awesome tsundere who has a nice compatibility with Ayumu... yet it's still hard to get the "onee-san" image out of my head.

I used to dislike characters who appear too young, but I got used to it. Once I did, I liked Shana a lot more; her atmosphere is mature. Ah, but after reading the Zero no Tsukaima light novel, I realized that the anime character design of Louise had ruined my sense of romance... she's more beautiful and charismatic in the light novel, yet she comes off as cute and immature in the anime.

Kyoto Animation went out of their way to make Itsuki younger and Mikuru more beautiful for the second season of Haruhi, and I wish other anime studios had the same courage/humility.

Edit: Oops, this wasn't meant to be a reply to a specific post within the thread.

This. I used to love DNAngel. thought it was top stuff...when I was about 10/11 about 8 years ago. I re-read it a few years ago and realised 'wait a minute...I'm really expected to believe that a bunch of 12/13/14 year-olds are running around and doing all this?'
This was also what I disliked most about Gungrave. Big Daddy and Maria's daughter, Mika, is only 12, I think. But she's made to look and act around 16 at least. Her being 13 just made her seem like a whiny brat (granted her parents were killed >.>...but you know what I mean. And I realise the anime was adapted from a game so it's not the anime creator's fault).

Guess I'm a little late to this one. Yes, I agree that it can get annoying at the extremes, especially with some of the loli/shota stuff I've seen, but most of the time it doesn't bother me much. Perhaps it's partly that while I'm in my mid-thirties, most people think I'm much younger. As in, if I'm out with my teenage nephews, people assume I'm under 18. And I don't wear makeup (just sunblock), and I dress in business suits most of the time. I don't speak like a teen, I don't dress, drive, or act like a teen, but I still get carded left and right... what the heck?

And about 12 year olds looking like adults: does this count? I, at age 25, with my younger brother under 20, went to a restaurant. The waitress offered my brother the wine list and asked me if I needed the kids' (under 12) menu. WTF?!?!