Since it's now known that the human brain isn't completely finished maturing until early- to mid- twenties; and since I, personally, think people should spend at least a few years living as a grown-up before having children; I think someone should be at least 25 (provided s/he is as mature as someone that age ought to be). I think guys should generally be older than that.
People all think differently, of course; but I was 25 when I started to adopt my eldest son (but it was an unusual situation that occurred, rather than something I planned on). I was late twenties/early thirties with the two younger ones. Their father was early/mid thirties. I like having done things that way because he and I both had plenty of years to be young adults without yet having a family's worth of children and responsibilities. It pretty much looks like my two sons and daughter will do things the same way (have that chance to live as young adults for a "solid" length of time before becoming parents).
At the same time, I got to stay a little younger just that much longer. (I still had kids in college when others my age had a bunch of grandchildren.) Also, now that my have grown, I'm having the chance to live as the parent of grown kids for awhile, without getting into the whole grandmother thing quite yet. I've got that to look forward to one day; but, for now, I'm getting yet one more chance to be an adult without a lot of the issues associated with having young children (or young grandchildren). I figure, just as with waiting a little while before building my whole family, I'll be nice and ready to be a grandmother by the time it happens.
Besides, I think parents who are good and mature and ready, and who want their children, often tend to be better parents than they might otherwise have been had they been a lot younger.
Having said all that, though - I don't think women should, if they have a choice, wait until their late thirties; because too many run into fertility problems if they do. Also, there's the concern about increasing risk of problems for babies born to older mothers.