I suppose there's no right or wrong age to stop, i love reading all these replies.
I guess mother nature does put a stop herself when menopause arrives, but then she does have a bizarre take on having babies when you remember she "starts" us all at around 11-13years of age so perhaps menopause isn't the best time to wait!
I have had my kids during my twenties and I'm about to reach 30. I'd still have another but wouldn't try for more after about 36 years I guess. It's up to the individual though, if you really want a baby at 40 and mother nature lets it happen, then why not?