If you're not running on instincts then you might want to be particularly careful with your beliefs in this area...
Peak fertility is different that the optimal age for a first child. Fertility is much easier to measure (based simply on the probability of getting pregnant given an standard opportunity to do so) whereas the best age to have your first child is a ridiculously complicated calculation having to do with your values and goals plus: the current and future state of medicine, the current and future state of the economy, your current and future pool of partnering opportunities, and probably other stuff as well.
Azathoth (who doesn't know about fertility medicine or transhumanism or the singularity yet, and was informed of the pill one or two "clock cycles" ago) probably thinks it is a good idea to be very fertile near the end of one's period of fertility because it's your last chance to have your last kid, even if the probability of birth defects is substantially higher.
In the modern democratic/industrialized environments, women don't have replacement levels of children. This might be "good" if we're all looking around and correctly determining that the population should be lower and 0,1, or 2 "really well raised" kids are better than 8 "poorly raised" kids. Alternatively, this might be "bad" if our parenting instincts are just going crazy in this environment. Like it could be that if/when we're well informed 70 year olds who resist cognitive dissonance we might look back on current reproductive decisions with justifiable regret.
In the (justifiably controversial) book The Bell Curve, the authors claim that before the advent of SATs, merit-based scholarships, and a universal college expectation for smart people, society was different in many ways, including that people in college were more likely to have rich parents but otherwise had the same intelligence as everyone else, and also that higher IQ predicted early marriage, early parenthood, less divorce, and larger total family sizes. I have never been able to find something peer-reviewed to back their historical claims, but it's one of those head scratchers that make me wonder sometimes about the larger socio-demographic picture and whether there is some kind of mass craziness going on with respect to family planning in WEIRD countries.
One more factor-- I think people are less likely to have children (or many children) if they trust that larger social structures (private and/or public pensions and provisions for care) will support them when they get old.
I believe that WEIRD (and we probably drop the "white" because the meme definitely spreads to other races) cultures are unsustainable at present tech because the birth rate is too low.
Daniel Dennett has advanced the opinion that the evolutionary purpose of the cuteness response in humans is to make us respond positively to babies. This does seem plausible. Babies are pretty cute, after all. It's a tempting explanation.
Here is one of the cutest baby pictures I found on a Google search.
And this is a bunny.
Correct me if I'm wrong, but the bunny is about 75,119 times cuter than the baby.
Now, bunnies are not evolutionarily important for humans to like and want to nurture. In fact, bunnies are edible. By rights, my evolutionary response to the bunny should be "mmm, needs a sprig of rosemary and thirty minutes on a spit". But instead, that bunny - and not the baby or any other baby I've seen - strikes the epicenter of my cuteness response, and being more baby-like along any dimension would not improve the bunny. It would not look better bald. It would not be improved with little round humanlike ears. It would not be more precious with thumbs, easier to love if it had no tail, more adorable if it were enlarged to weigh about seven pounds.
If "awwww" is a response designed to make me love human babies and everything else that makes me go "awwww" is a mere side effect of that engineered reaction, it is drastically misaimed. Other responses for which we have similar evolutionary psychology explanations don't seem badly targeted in this way. If they miss their supposed objects at all, at least it's not in most people. (Furries, for instance, exist, but they're not a common variation on human sexual interest - the most generally applicable superstimuli for sexiness look like at-least-superficially healthy, mature humans with prominent human sexual characteristics.) We've invested enough energy into transforming our food landscape that we can happily eat virtual poison, but that's a departure from the ancestral environment - bunnies? All natural, every whisker.1
It is embarrassingly easy to come up with evolutionary psychology stories to explain little segments of data and have it sound good to a surface understanding of how evolution works. Why are babies cute? They have to be, so we'll take care of them. And then someone with a slightly better cause and effect understanding turns it right-side-up, as Dennett has, and then it sounds really clever. You can have this entire conversation without mentioning bunnies (or kittens or jerboas or any other adorable thing). But by excluding those items from a discussion that is, ostensibly, about cuteness, you do not have a hypothesis that actually fits all of the data - only the data that seems relevant to the answer that presents itself immediately.
Evo-psych explanations are tempting even when they're cheaply wrong, because the knowledge you need to construct ones that sound good to the educated is itself not cheap at all. You have to know lots of stuff about what "motivates" evolutionary changes, reject group selection, understand that the brain is just an organ, dispel the illusion of little XML tags attached to objects in the world calling them "cute" or "pretty" or anything else - but you also have to account for a decent proportion of the facts to not be steering completely left of reality.
Humans are frickin' complicated beasties. It's a hard, hard job to model us in a way that says anything useful without contradicting information we have about ourselves. But that's no excuse for abandoning the task. What causes the cuteness response? Why is that bunny so outrageously adorable? Why are babies, well, pretty cute? I don't know - but I'm pretty sure it's not the cheap reason, because evolution doesn't want me to nurture bunnies. Inasmuch as it wants me to react to bunnies, it wants me to eat them, or at least be motivated to keep them away from my salad fixings.
1It is possible that the bunny depicted is a domestic specimen, but it doesn't look like it to me. In any event, I chose it for being a really great example; there are many decidedly wild animals that are also cuter than cute human babies.