That won't happen on voting, 18 yrs old for voting is specifically mentioned in the constitution.I'm willing to give into the 21 year old thing if we do the same for voting. If 21 is when you can exercise one right, then 21 should be the age you can exercise all of them.
But, other things not mentioned by age, in the constitution could, I suppose? 21 for drinking, 21 for drafting, 21 as the age of consent, etc??????