For me it's more a case of what stage in life they're at. I'm at the point where I'm starting to build up my career, and will be looking to settle down in a few years, and then kids after that.
If somebody is going to be going off to uni, then looking for a job etc. then they won't be looking to settle down for another 5 or 10 years.
Equally if somebody is already settled down then I feel like I'd be missing out on some of my years if I settled down with them.
Unless she's like 12 or something, then you should be ashamed for even asking.