Is it okay to ask a 16 yr old girl out even though I'm 20?

If you honestly care about her, the respectful thing to do is wait until she is legal to be with (wherever you live). Or ask her parents for permission, or just keep things friendly. If you do ask her out and you guys begin dating, be very careful not to take advantage of her because 16 is a vulnerable age and she hasnt fully matured yet. If you care about her, you dont want to hurt her by going too fast.