You should also ask if the people who learned new skills were raising kids. I have a feeling that is the dominant confounding variable.
Having kids forces you to learn new skills, at least for me since I had 0 experience "growing kids from 0 to 20".
Besides the skills to keep them alive, teach them to walk, talk, behave, read, get dressed, keep clean, be safe, etc. There are: picking the right neighbourhood for young families, finding the appropriate schools, dealing with friends of your kids and their parents, planning interesting holidays, adhering to all the mandatory festivities at the corresponding days.
If that does not count as leaning new skills, I do not know what does.