I do not think that having children - I have three teenagers - keeps you young. The reverse. It thrusts you into a full-frontal confrontation with your own all-too-obvious maturity.
I've never regretted not having children. My mindset in that regard has been constant. I objected to being born, and I refuse to impose life on someone else.
Appallingly, I hadn't thought about it one jot. I never daydreamed as a little girl of getting married and having children. I was as surprised to discover I was getting married as I was to discover I was up the duff.
There are all these things I want to do when I don't have to finish a book. But I have to keep writing because I keep having children.