It really depends on how old YOU are.
Example: three of the four people I've dated since high school have been six years older than me (don't ask me why it worked out that way; I think it's weird too). It went like this:
Relationship 1: I was 18, other person was 24 (lasted 1 year)
Relationship 2: I was 19, other person was 25 (lasted 3 years)
Relationship 3: I was 22, other person was 23 (lasted 1 year)
Relationship 4 (current): I'm 24, other person is 30 (has been 9 months so far)
Relationship 1 was a spectacular failure because I was fresh out of high school and had only had a glimpse of the real world, while my partner was getting ready to settle down and have a family. It was actually really creepy (this person became interested in me when I was 17 and loved my innocence more than anything else), so the less said about it the better.
Relationship 2 suffered from the same problems because I was in college and wanting to experience life, while my partner was working and waiting for me to graduate so we could...settle down and have a family. The whole relationship was based around waiting and eventually neither of us could wait any longer and thus parted ways.
The third person was entirely too immature for me, despite being a year older than I was.
And now there's my current relationship. I have graduated from college, had all the youthful experiences I ever wanted, and am sick and tired of of dating relationships, so I'm finally ready to settle down. The other person feels the same way, and we're working out very well so far.
In general, while you're in school it seems best to stick with people about your age. (High school students should date other high school students, college students should date other college students.) After you graduate, you can widen the age gap a bit.
It's not just about maturity, as I have always been mature for my age, but about what you want out of life at the stage you're currently at. It's always best to date people who want the same thing from the relationship as you do, and it will be much harder to find that if one person is in school and the other person isn't (or one is in high school and the other in college).