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).