I'm 15, and here are my thoughts...
A younger character can be annoying, but is usually quite interesting. An example of this would be "The Boy In The Striped Pyjamas" or "I Am David" - both younger characters, who sound younger but not thick if you get what I mean.
Same-age: I don't like books about everyday life, such as the Mates, Dates books. They get on my nerves! However, there are plenty of good books with a character in their teens such as the Pellinor series, "The Curious Incident Of The Dog In The Nighttime" (about a 16 year old ... I can't think of the right word....I keep thinking alzeimers but it's not that....well it's good, anyway) and His Dark Materials.
Adult characters: I like them as long as it's a good plot, story etc. But they can be boring sometimes. It doesn't bother me that they're adults, but it does mean that there can be better "real life" stories, because more is happening to them. I especially like "My Best Friend's Girl".