[AnimalMuppet]

Sure, they can change their mind and become a biologist. But in doing so, they throw away the two years. They have to start college from zero, because their trade-school classes don't translate into college credits.

You can start college at 20 as easily as at 18. The problem is that you get nothing for the two years (except, I suppose, you can work as an electrician to pay for college, instead of working at McDonalds).

[doorstar]

I'm guessing that our hypothetical student must or might take a college-level Biology course as part of their undergrad degree, and that exposure might cause them to change their mind about their chosen field.

I also don't know how easy it is to get into the same college at 20 that one would have gotten into at 18. If at 18 they got accepted to a school that cost 100K/year (as the parent comment suggests) then they may find that school closed to them 2 years later.