It really depends on the girl. It's stupid to have a magical age that people are suddenly "dateable", "sexable" or "mature enough". People mature at different rates depending on experiences from their past. Just get a feel for the girl before dating her. If you think she seems immature than don't even consider it. But if she suprises you with how adult she is, perhaps it's worth a try.
Aside from that though, 16 is a weird age. Some people might look down on you for dating someone who isn't 17 yet...not to mention that 16 is not legal in some states. It might be good to just wait till 17 to look better on paper
...then again, if you're 20, the difference isn't big enough for it to matter that much imo.