I have found that I often go with whatever direction the other players haven't. Like, if we are a group of old codgers I tend to be the dick who plays like a 17 year old, and vice versa. I think I typically play characters around the age range of 25 -35 because I can understand the thought process of them easier. I've ran the gamete though. Gotta serve the plot and sometimes certain ages don't work.

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.

Younger. Physically, I'm 26, but mentally I feel like I'm 18. xD

When I started, I RPed older folks; usually tried to be the voice of reason, caution, calm, and forgiveness. Typically backed-up with broad skillset already mastered.

But as years went by I decided to get reckless and RP younger characters for an added spike in difficulty as those characters had to learn things I as a person already knew, typically the hard, painful, and often in embarassingly-amusing ways.

This has kept my median roleplay-age somewhere around mid to low twenty-somethings unless RPing races that live ridiculously long (or short) lives.
-With exception to the wheelchair-bound 75 year old badass-grandpa in power-armor.

I play a lot of fantasy monster characters who run 200-2k years old. In fact, most of the characters I am currently playing are at least 100 years old.

When playing boring humans I usually pick early to mid-thirties. Maybe forties. I rarely ever play anyone younger than 25 and I never play children unless they are NPC/side-characters who don't interact with the main characters much at all. Most of my RPs have mature themes and I have a strict "no pedo" rule that includes RP partners who are under 18 (sorry, kids.)

Depends on what helps the story or the character more.

Old people are good for conclusion and revelations. Young people are good for discovery and learning. The difference between the two at the core is justification: Young people know jack shit, even when they think they do. They can be smart and right...but still know jack shit. Because it's all based on theory, rarely practice. One again, that dosen't= wrong. It dosen't means that, it's all fine to write a paper sending a rocket to the moon.

Different sending a rocket to the moon.

Old guys are justified in their conclusions...but they can still be wrong. That real world application dosen't equal a working life. You can be right that swindling old folks makes you rich, dosen't make you any less of a cunt nor does it make you any less unhappy with your lose of integrity.

I like old guys because typically old guys are good as player to GM interims. They know about the world, even have regrets about the events in it. It's a good way to "rewind" bringing other characters who may not have motivation back with you. Making them see events, judge a personal morality and see the story in it while making them pertinent because...hopefully...my character would be pertinent to them.

but hey. I like being a young guy who hunts big titty anime girls too. So, tomatoe, potatoe.