Do you consider 21 a milestone birthday?
DD is 21 soon, and many of her friends have already turned 21; none of them has had a party, and they have all just marked the birthday like any other... they all consider 18 the milestone coming-of-age birthday, and I would agree.
I'm old enough to remember my aunt turning 21 in 1969, before the voting age was lowered in 1970.
YANBU 21 is not a milestone
YABU 21 is still a milestone