One thing I've always been good at is writing from adults point of view. It's probably because I spend so much time around them; only children tend to be more comfortable around adults, and I'm no exception. During a volunteer job I spend a lot of time at the Senior Center in our town, and so I'm very comfortable around older people. My dad has a lot of business trips and I usually go, and at the receptions and dinners I'm generally the only fourteen-year-old, so I've developed a skill for conversing with people in their thirties, forties, and fifties. I think I could probably pull off a businessman in his forties pretty well.
However, the difference is less noticable in fantasy, because while characters are definitely important, a lot of fantasy books tend to be more plot-driven. Also, in an alternate world, the twenty year olds aren't dealing with life in a tiny apartment and the forty year olds aren't dealing with putting their kids through college. Instead, they're dealing with battling the dark overlord or whatever. (Although I hope your story is a lot more original than just that.)
Still, because I tend to think of myself as writing young adult fantasy, I never let my characters get too old and I usually have one protagonist whose a bit younger. In the novel I'm currently revising, for example, I have four POV characters: A sixteen-year-old girl, who comes across as very naive because she's inexpirienced, two fire-elementalists, both in their mid- to late- thirties; my oldest POV character is the king of one of my countries, and he's thirty eight. I'd say that my supporting characters are pretty even as far as younger ones (say, 14-20) and older ones (say, 30 and up.) Oddly enough, I have very few characters in their twenties.
But anyway, I want this to be a novel that young adults can read, which is one of the reasons that I'm going to keep the 16-year-old a POV character in the planned sequel. There are many other characters who's heads I want to get inside, but if I didn't have her, I don't think I would be able to say it was young adult fantasy anymore, and while it wouldn't be any different, the thought of writing adult fiction sounds a lot scarier.