I guess I'll elaborate on what I said before: I was 20 when I took the LSAT. Looking through the Yale Law admitted student boards, quite a few people are 21. I've heard of people as young as 16 (admittedly, an outlier) getting into Yale.
I believe, based on my observations from the past two years, that being under 20 at the time you start law school can negatively affect your admissions cycle. However, it seems like 20 is a magic number -- I've never seen someone demonstrably affected by their youth at that age. You're of course free to judge for yourself how representative what I've seen is -- just throwing it out there for you to consider. It seems that as long as you don't give admissions officers a good reason to question your maturity, they generally don't.