Midnight Llamaman said:
As long as you're happy and legal, who cares.
It's way more complex than that.
This is going to sound obnoxiously arrogant, but on average (as with everything, there are exceptions) a 22 year old is going to be more emotionally mature than an 18, 19, and often even a 20 year old.
I date based on emotional maturity at this point. I dated a girl last year who was 19, but had the emotional maturity of a 16 year old. I didn't see it at the time, but I grew to realize it. I'm never dating a girl like that again.
Contrary to that, there's a girl I know right now that is 18 who I would go out with, as she's more mature than your average 18 year old.
So to more directly answer the OP... I would very, very rarely date a girl under 21 at this point. But I would never, ever date a girl under 18. That's, you know, illegal and creepy.
EDIT: I also probably wouldn't date anyone over two years older than me. I'd like to go through life on a more even playing field. You know, go through life's changes together. I know, it seems like a bit of a double standard there (I forgot to mention I was 22 myself) as I would date an 18 year old depending on maturity, but think about it... it's cool for an 18 year old to be more mature than their age. But for a 26 year old to have the maturity of someone around 22-24? Eh... not so much.
But like with everything, there are exceptions.