We live in a society where the divorce rate is 50% and climbing. Years ago there was no divorce. People married in their late teens, there was a defined head of the household who had final say, and you had to deal with things. In the last 100 years things have changed. Equality means there is no one person with final say but as a society we never learned compromise. Medical miracles extended our life expectancy giving us more time to spend in school and developing profitable careers before establishing a home and growing a family.
But basic biology did not change. Puberty, hormones, attraction, and the desire for a mate is the same now as is was centuries ago. Maybe off by just a couple of years ... instead of 8-11 we're now at 10-13. With that in mind, the desire for a mate and home and family at 18-19 is perfectly normal.
That said, what I'm teaching my kids is the difference between subsistence living and and higher-end material success. They can start a family in their teens, get jobs, and make it just fine. Working at minimum wage or learning a trade. And prettty much living paycheck to paycheck or very frugally for life. OR ... they can hold off, go to school, start a career, and start a family in their late twenties. Earning 50% more, having jobs or professions with upward mobility, insurance, and vacation plans. Standard of living skyrockets.
Happiness isn't dependent on income though. My 5 year old has just as much fun going to the park and community pool as she does at Disney or a cruise. It's different for me though. I want the cruise! If your kids are set on this course, I would explain the economic difference in waiting, and then support the decision they make. Because in the end they are going to do what they want. You just get to decide how happy you can be with it!