I'll be having my 12th kid soon. That simply doesn't happen if you wait. At the start, I had no idea that I'd want so many kids. To allow for this possibility, you must start early. Ideally you'd start before age 20.
In college? Or high school?
I think it is important to have some means of supporting a family, but that might not involve the educational path that has become our standard for the past 30 years. It only takes one good income, and paid child care is usually a terrible deal for big families, so there isn't a financial justification for paying for two expensive educations. If the degree costs time or money and wouldn't get used, why bother? Avoiding unneeded student loans is a sensible idea. The earlier you start, the larger your family can be.
The lines between college and high school are starting to blur. Dual enrollment (or equivalent) is offered in most states. This lets students take college classes early, getting credit for both high school and college. Here it can start in 6th grade. You can get a BS degree before completing high school.
The reasons to wait are legal (typically must be 16 to marry), medical (roughly similar), and the ability to find a suitable spouse. That last one is usually easier in college. It can be a challenge to get everything in order at a young age, but the payoff is huge.