Not everyone has the advantage of the spouse boarding in zone 1 and getting everything settled so that other spouse can board at end.

as for storage it is not just about space but wanting it have it close at hand. Those without kids should have ni issue if their carry on is a row or two away.

but it is more about settling in, getting out what may be needed during a flight eg diapers, wipes, snacks, bottles, little toys etc.

As one who has traveled with 3 kids under five I liked getting on first to settle everything in. Nothing worse then having crying child while have to get up and find stuff.

As fir for six year olds we are not talking about them. It is about those with young children - usually under two I believe is the cut off