I think it depends on individual maturity levels.
A friend of a friend I know just had a baby @ 28 which a lot of people would consider to be a perfect motherhood age.
Yet, this girl is an immature, totally self absorbed drug attict and alcoholic who in my opinion will probobly not be a good mother. *ahem*
The first friend is smitten with her and refuses to believe that she'll fail miserably.
I am occasionally in a position where I have to deal with her and use this time to observe and document, since there is a precious child at stake. (She drank through entire pregnancy.) *sigh*
My point is, I didn't vote b/c there is no right answer.
My own mother began having all of us at 18 and did remarkably well, in spite of the sperm donors refusal to contribute time or money. *shrug*