A lot of people I know become not just mellower but nicer as they age. (I sense this, perhaps, most especially in novelists -- where main protagonists are obviously more patient, sweeter, to other characters in later books than they were in the novelists' very vibrant but more charged and angry earliest works.) I have some suspicion that what happens with those who have self-esteem-enriching experiences of validation and attendance when they were young, but also hampering experiences of abandonment and sadistic treatment, is that they still have it in them to acquire more of what they were lacking and deal with some of what has tended to haunt and stop them, while they go through life. This may in fact be -- without them being consciously aware of it -- what a great deal of their life endeavors are mostly about. And if they end up getting some of the attention they were needing, learn not to denigrate but work to satisfy their own needs, they no doubt end up being better able to attend to their children when they have them than they would have been if they had had them when they were younger. That is, even if the seed is worse, the DNA somewhat hampered, the story of the unfolding and development into its final psychogenetic form may be a better one with older parents.
My mom is a nicer, more giving person than she was when I was a teen: she listens better, more generously, than she once did, and conversations with her leave me feeling warmer and more optimistic. She has largely satisfied her need to be the career woman, a pursuit which left us feeling like our own ambitions were of secondary import when we were teens. I wonder, given how important the quality and quantity of attendance is to the emotional/intellectual development of children of our species, if we should be looking more to the best PSYCHOLOGICAL age and less to the best biological age, for having children?
In any case, this is vein to be mined. Not just because real rightness will be discovered there, but much needed fairness too: as Vanessa argues, if you're in your 30s, without kids, and not obviously on a professional path, you will be looked at as if you are the runt of the pack. Conversely, if you are professional, late 20s, and have a child or two, you are being everywhere "told" you shine golden -- whatever the actual degree of dullness of your story.