Modern healthcare took care of that as you say, what's the infant mortality rate down to these days? and also the likelyhood of a mother dying during birth? not very high. this is reflected in the low number of births needed per couple to insure that the population doesn't shrink.
Regarding insuarance companies, they rely solely on statistics, have you tried getting a car insurance when you were under 25? crazy expensive, just being able to untick the box saying you have a driver under 25 driving your car will cut your insurance in half (at least in my experience).
Giving the younger people a chance to try and fail to live on thier own before they are 25 shouldn't really be a problem. you will still be there but it could be an important part of thier development.
I think that 18 years of development would be enough and give the kid a chance the last few years to live in their own. the point of raising them is to make the kids into something that is able to live in it's own.