I've had my 2 and I am done - I was 30 when I had DS2. I suppose that makes me one of the younger ones in this group, which feels odd as I didn't consider myself a younger mum at the time.
I've been friends with a kid of older parents (i.e. 10yo living with a 50yo and 60yo) and it did feel a bit like she lived with her grandparents. They were relatively strict though, so that's a confounding factor. She had much older siblings too, so always felt like a bit of an afterthought in that house. I felt sorry for her and it put me off the idea of having kids late, rationally or irrationally.
I do think there are things to be said for starting early if you want kids and it's feasible - I have retrained post-children and have the prospect of 30 years in my new field ahead of me. My mum had me early, DH's mum started very early, DH's stepmum couldn't have kids as she started trying late (early 40s - it didn't work for her) and it is a major, often vocalised, regret of her life. All of that, plus a scientific background similar to aurynne (whom I agree with re:all the facts) made me very anxious to maximise my chances by trying to have children early. I was fortunate enough to have a very easy time of it, biologically.
I'm actually very lucky I did, because it transpired that my mum (who was very very close-lipped about any biological matters) went through the menopause proper at 45 and her sister at 40. I sometimes wonder what would have happened if I'd waited and shudder slightly. I'd encourage all women in general to ask their mums about their menopause date if possible, because that's useful knowledge to have.
I like a previous poster's comment of 42+18 = 60 - that seems as good an arbitrary cut-off point as any 