DO's are for everybody. Although the < 21 crowd is pretty small, I do recall some 18-20 years at CO DO II (didn't attend I or III) & the NW DC-10 Retirement. They fit right in. I wouldn't worry about it.

Most dinner activities are at restaurants that also serve alcohol, rarely at bars. The bars we do spend time at tend to be in open areas in our hotels -- as long as you don't put yourself in a position to be asked for ID, you're unlikely to be hassled.