If you want to participate in parenting, you'll want to be somewhat fit. You can expect a lot of lifting, carrying, squatting and sitting on floor, stroller pushing, running after etc. Better chance you won't get exhausted by that at your thirties than fifties.

The older you are, the more likely that you will pass genetic abnormalities to your child. Sperm gene quality (i.e. the genetic version of not having bitflips in memory) deteriorates with age. If you have a choice it's probably better to have a similar timetable as women