My bf turns 30 next month, I turn 24 in March... Our age gap is quite nice, because his mind frame is still someone in his mid-twenties, rather than a 30 year old. Not saying he's immature (he isn't), but has all the 'settled' things like career, lifestyle, beliefs down, but is willing to wait awhile still before children and owning a home comes into play.
I was engaged to a guy who was 8 years older than me (I was in my late teens, he was in his late 20s). This was a HUGE mistake, and is why I caution those girls I see who are 18-20 and considering marrying someone in their late 20s. You are still working through all the things you want as you get older (believe me, you may THINK you have it all worked out, but you don't - it changes a lot more in your teens, than it does when you get older and in your mid 20s), whereas he's all ready gone through all that, and is more often than not pretty made up in what he wants, and is ready to settle down. This can cause some problems for the young women, as they end up completely deciding their future based on what the man wants, rather than what they themselves really want.
So, for those in their late teens, an age gap over 4 years would be highly dangerous to themselves and to the relationship (I've yet to see a mature, stable relationship between people of these ages) - a lot of co-dependancy traits seem to manifest themselves more, with this factor added in, than in other situations. Once you're around your early-mid twenties, an age gap around 5-6 years isn't so bad. Once your in your late 20s, age gaps (I'm assuming the youngest being in their late 20s here) don't seem to cause much of a problem - provided fertility isn't an issue.
A lot of this is dependant on maturity levels in that age group though, I've seen lots of mature 18 year olds - but I still think they should wait until early/mid 20s before making a decision as important as marriage - that stage of development is huge, and can be greatly influenced by a decision such as getting married. I see far more benefit in waiting out that adjustment phase (ie school-uni-out in the world) and getting seriously involved with someone AFTER that adjustment phase has been completed. Doesn't matter how mature you think you are, development still happens a lot at this point (pretty much more than at any other stage in your life), and I think it's dangerous to be working through that, and attaching yourself to an older male at the same time.
I've seen a lot of immature 30 year olds too, and that goes beyond development stage of life. So a big age gap (ie more than 4 years) is far more dangerous when you are in your late teens, than it is as you grow older - maturity levels end up balancing out over time, from what I've seen - most girls mature a bit earlier (psychologically in particular) than guys do, so a girl who is 24/25 can have quite a healthy relationship with someone in his late 20s/early 30s.
Just my 2 cents!
Sasch