This is an old one, but still relevant today. The draft still exists. The Selective Service Act requires all males, beginning at age 18, to register for the draft. If one is old enough to die for their country, they should be able to vote. Taking a young person's ability to vote may take away the pride and civic duty they feel for their country. Permission to vote alongside the registration of the draft instills major responsibility, one that can't be understood by a sixteen-year old.
Although the draft still exists, it's not as relevant as it once was. Perhaps it may be in the future, but in today's terms it makes no sense to base the minimum age to vote after the age of draft registration.
[P1] Registering at 18 for the draft is a declaration of patriotism. [P2] Voting is an act of patriotism.
Rejecting the premises
[Rejecting P1] Registering for the draft is not a declaration of patriotism. [Rejecting P2] Voting is a right, not an act of patriotism.