I don't have any sort of difficulty getting into the heads of characters far younger than me or far older than me (I'm fairly old by the standards of role-players, from what I can tell), so realistically speaking I don't have a preference in terms of ease of use or being able to handle the character. My main character is within my own age region, starting out in his late twenties (so younger than me, but keep in mind the story spans more than a single year so he does get older than he initially is, and there's flashback arcs too). Overall though, I've got a wide range of characters, within pretty much any age region at all. I really don't feel that I pick the age of a character to contradict or counterbalance those of other characters in a story, but rather I just go with whatever suits what I need from them.

With that said, if I've got any sort of actual preference for a character type - in terms of enjoyment, rather than ease of use - I'd say it's characters with wisdom and strong emotional maturity, so I lean towards characters who're either a bit older than the norm (15-25 seems to be the norm) or are 'wise beyond their years'. Of course, this is just because I relate to those types more, subsequently tending to prefer characters within the late twenties to early thirties region as a general rule, but I'm fine with playing some naïve youngster or playing a character far older than I am, but only if I find a story that offers me an engaging way to use that role and I come up with a character that I personally find interesting. It really depends on the character themselves.

So yeah, I have a preference in terms of enjoyment and personally relating, but not in terms of ability to get in their head.