I agree. I retired at 38 after 20 years and I can't imagine a 39 year old in basic training keeping up with an 18 year old. Not to mention taking orders from someone young enough to be your son. There are exceptions but on the whole I think this is a bad idea. Old bones don't heal as fast as young ones.
I think that would be the tougher part of it. There’s a percentage of 40 year olds that can perform like they were 20, but everyone of them will have to seriously psych themselves into working for 20 year old authoritarians.