I don't drink, so instead of going to the bars for a drink at 21, I applied for my license instead. So needless to say, a 21 year old is totally fine carrying.
I recently turned 21, and I did the exact same thing. I agree with everyone who has stated it depends on the person and their maturity level. I've come across some friends and family that support it, and some friends and family that don't. Ultimately, the people who support me in obtaining a ccw and choosing a handgun (I'm still in that process, I've decided I need to take some extra time to go through training and try many many guns before making a decision) are the ones that know me best. The people who aren't in support of it almost always know very little about guns or gun safety, and would be against it regardless of age.