In general, you’re correct. But it is completely arbitrary that you can’t legally drink at 20, but can at 21. Hell, in the past, the legal drinking age was 18. Personally, I think we should go back to that.
Traffic laws are similar. Some states have a speed limit of 65 on highways. Some have a 75 limit. Regardless of what that limit is, if you break the speed limit, no one is going to care that you’re capable of driving at a higher speed.
A voting age of 18 is a completely arbitrary number. Plenty of kids who are younger are capable of doing the research necessary to make an informed decision. Plenty of aduls who are older are completely uninformed and vote purely based on party affiliation. We don’t give intelligence tests to determine who and who can’t vote and at what age, we have a one size fits all policy for voting. It’s the same with an attendance policy.
If a student doesn’t like a mandatory attendance policy, take a different class with a different professor. If all classes have that policy, then maybe try to get an individual exception. But if you can’t get an exception, or just don’t show up, and don’t have a reason, don’t expect the rules to not apply to you, regardless of your ability level.