This probably depends where you live, but one under 20 would be making the best of a contraception fail, two a bit careless unless they're twins obviously.
I don't know many people who've had a child under 20. The one I do is every bit as good, or otherwise, a parent as anyone of any but she gets a lot of help because everyone assumes it's hard for her and her partner and people want to support them in finishing their education and starting their careers. Neighbors and wider community do more free childcare for her than they'd think of doing if they were 10 years older. She had her son at 18 a couple of months after taking A level equivalent, her partner was the same age but left school and got a job, they live with his parents.
It probably depends whether a flat of your own is affordable for a couple under 20 in your area. Where I live hardly anyone can afford accommodation for a family at that age and 20 year olds are living with parents or in house/flatshares. That's probably why it seems an odd choice to have not one probably accidental pregnancy but two children by 20 to me.
If you've got everything sorted and a 3 bed flat or house and jobs and don't live with or off your parents then more power to your elbow and all that, good for you.
There certainly are things you can do at 19/20 but can't at 40 though - 3rd class ladies carriage sleeper across India, sleeping on floorboards in illegal dorms in Vietnam, staying up working for 48 hours straight fueled only by coffee and writing a dissertation actually worth a first class mark in one sitting, cramming for exams all night and actually being awake and clear headed enough to write good answers... Not everyone wants to do those things of course...