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There's also a lot of societal pressure to wait. My wife got some surprisingly nasty comments from close friends and family members when we found out about our first (we were both 22), and continues to get them from some people now that she stays home with our three kids (I won't even go into the questions regarding whether we're "done" having children).
Personally, I'm glad that we've had our kids "early" a) while we have the energy and health to look after them and b) so that we can enjoy our 40's and 50's in (relative) peace and quiet in our own pursuits (and so that we'll still be young enough to enjoy any grandkids!).