This would be a sea change. It would mean the chairman of the joint chiefs would be in his mid-forties, instead of 62, as he is today. The career path would be compressed to an astonishing extent. It is not just generals who will be (much) younger; it will also be every type of superior officer.
There are, of course, laws in America that ban age discrimination. There are very good reasons for these statutes, and I don’t propose changing them anywhere — except in the military’s officer ranks.
Now, you might be concerned that my plan will promote people past their level of competence. Don’t worry about that. Bureaucracies promote people to their level of incompetence; startups and adventurous militaries promote people past their level into competence, into jobs that they must grow into as they do them. A very common feature of military campaigns, especially successful ones, is officers being promoted very early due to high rank turnover, whether due to dismissals or death in action.
Plus, the U.S. military needs more accountability for senior officers. Some senior officers will not be ready for significant command by their mid-thirties. Replace them!