I fully agree. Though the exception for the military would probably set up a false, but convenient, "equal protection" argument in courts for many years.
I'd settle for 21 across the board. At least most people who've attained that age will likely have some real-world working experience to know how economics and handling money feels first hand.
So your theory may not apply, but there is always some Judge who issues an injunction to say otherwise nowadays. Obama ignored many; Trump doesn't.
I'm still flabbergasted (I'm that old -ha)that it took a USSC ruling to allow the Commander in Chief of the Armed Forces to ban mentally disturbed transgenders to serve. Believe there is one lawsuit still outstanding, but with the Supremes ruling, it is going nowhere.
While the Founding Fathers could not foresee the future and could have been more specific on certain issues, the Constitution couldn't be more clear that the President/CIC has ultimate authority over the Armed Forces. That said, I'm not good with the Congressional "War Powers Act" that gave the President the ability to invade without a declaration of war.
Sidenote: Once Congress funds the military, I believe the CIC can use the funds for any national security reason. Think WALL!