Reason for 31.
20s are a really enjoyable time to be free, apart from work, to enjoy life as your disposable income will be at its highest with no dc. Also lots of energy, and it's an 'exciting' time dating wise. Also gives you a chance to save so that when you're earning less with young dc, you get some passive income.
Late 40s (I'm in them now) - I'm knackered. So much tireder generally.
So, the main work is 0-17. I'd take 31-48.