I'll let you in on something that probably isn't a secret: I read more than I write. It's my form of entertainment; a way I can 'veg out' and leave the real world behind. Many times, I'll find inspiration in another author's work or maybe I can take what has been written and wonder how I would handle a similar situation. When I'm reading, I have a number of pet peeves. I'm going to tell you my biggest one.
Every story requires a certain suspension of disbelief. It's the price you pay for reading fiction; you know it's not real so you 'bypass' some things that are obviously unlikely if not impossible. I feel the author is responsible for not breaking that suspension by not doing anything so outlandish that there's no way it could possibly happen. I have a very difficult time reading a story where the author goes so far that there's no way I can keep that disbelief from creeping in.
The most obvious way that an author can break the suspension is through dialog. If you're character is a 14-year-old, he should probably talk like a 14-year-old. (That's one of the great things about the Harry Potter series to my mind; book one talks about like you'd expect an 11-year-old to talk and as the series progresses, the conversation gets more 'adult'-like.) His maturity level should be the maturity level of a 14-year-old (unless you prove he's a super-genius - but even then, believe it or not, a 14-year-old super-genius talks quite a bit like a 14-year-old non-genius because language skills improve with age - you don't just wake up one day and start sounding like you've swallowed the SATs). This means that there is likely no way in hell a 14-year-old is going to say something like '...I tacitly agreed to another meeting...' or '...to put it in perspective...'. Now, I can buy that one or the other might be used - but both together in the same conversation? It's not going to happen - and it intrinsically breaks the suspension of disbelief.
So, that is my biggest pet peeve. Specifically, having an x-year-old character who doesn't speak or act like an x-year-old character. If you don't know how an x-year-old character speaks or how they act, you probably should go and be around x-year-olds to study them - it'll make your writing so much better!!