I was reading only this morning about difficult it is for Millennials to move out of their parents houses, so I'm thinking new parents will become older in many cases.
I was 27 when I got married. That was an age considered relatively old to get married then but relatively young now. And yes I know one doesn't have to be married to have a baby, I'm just using this as a yardstick to different times different perceptions.
As what my mum called 'a career woman' I would not have considered having a baby before 30 and I've always believed it's best to go do stuff before starting a family if you actually want to go do stuff.. Obvs that's personal.
So when did I have a baby? I was 43, DH just a year older. Yes, it was our decision, but no it's not selfish. We wanted one child we could give a comfortable life to and not to do stuff first. It would have been selfish to have a child earlier and try to do the stuff and risk resenting not doing stuff.
Regarding energy etc - a super rude comment I got when I was a new mum was from another new mum (aged about 32-5). She said she could not imagine having a baby over 40 because I must be so tired. I wasn't then and I'm not now. Incidently, she was the one who looked tired and she's the one with grey hair and a worn look now.
Another thing I find scary is people being grand parents under 55, but, again, I know this is personal.
The answer is there is no perfect age in any other sense than what feels right for you at any given time.
PS - a colleague (male) became a dad at 64 recently