@EmeraldShamrock
Your post will be disliked by many. But I agree.
My DS can do 18 hours in one day full on sometimes
@Roozy123
It won't be liked her but what you have said is the truth.
Thank you both. 
I appreciate those comments.
Re what Emerald said... Kids (as much as we love 'em,) ARE a handful sometimes. Even when they are 'good and well behaved' (IYSWIM) they are still hard work sometimes. Demanding and energy-draining. Not always, but sometimes yep!
And they can be energy-draining, attention-seeking little feckers for a number of weeks in a row; several months even, (wanting attention 12 to 16 hours a day,) and it can wear you out. Children are wonderful, and we love them, and have great times with them, and many of us would not change a thing, but let's not pretend raising children ain't hard work sometimes.
Doing this up to your late 30s (looking after infant/junior age kids) is OK however, as you have lots of energy, and are fairly young and usually quite fit. I cannot even IMAGINE having babies/toddlers in my late 40s, and '4 to 8 year olds' at the age of 52 to 57-ish.
In the majority of cases, once you hit 42-43-ish, you do start to slow down physically, and no WAY do you have the same energy you had in your 30s (and younger...)
And just coz someone is 'pretty active' at 46, (as a pp said about their sister,) or they 'feel fitter than they were in their 30s.' that does not mean they can cope with the demands of babies and small children, (which can sometimes be relentless and exhausting.) Imagine looking after one or two small kids (like under 7,) when you are going through the menopause??? Nope! Not for me. No way.
Also, as I said, it's massively unfair on the child to have much older parents. As a pp said, (whose parents had her when they were in their mid to late 40s,) there were no real advantages but plenty of disadvantages.
These people really need to think of the child, and stop putting their own needs, wants, and desires first!