You can sit in a bar, at the age of 18. However, if you want to have an alcoholic drink, you need to be 21. Don't try to get a phony ID anymore, there are ways a bar can tell it's fake now, and none of them want to lose their liquor license. There may be an exception if the bar/restaurant are combined in one area.
Adults are those age 18 or older. Those of any age under 21 may drink. But a parent, guardian, grandparent, or legal-age spouse must give the alcohol. It is illegal for those under 21 to buy alcohol.