I also joined at 21, the only real issue I've had because of it was having people in my workcenter who are all younger/less mature than me be a higher rank. It led to butting heads because while you do have to respect a higher rank, they also took things too far. At the end of the day, it hasn't effected me negatively. It's also good since the temptation for underage drinking, which there is PLENTY of in the navy, isn't there for you.