As for feeling 20 but being older .............. were are all pretty much the same; I will be 60 in the not too distant future and I feel as though I'm still a young man. The thing is, it is when the body starts to tell you that you can't do 'that' anymore .... or you can, but you will regret it later. ('that' being almost anything physical that the brain says you can do)
Kids and the snip: I fathered two boys (planned) when I was 23 and 25 and had the snip aged 25 in 1975 as my wife was on the pill and forecasts were not good for long term usage at the time. It was good being able to go and play soccer/cricket etc with ones children, something that many mid 40 year olds are incapable of.
Enjoy your life now, you never know when you might not have it.
Good luck and best wishes.