I noticed this past week....the Air Force raised maximum enlistment age to 42 (officers and enlisted).
I pondered upon this.
Up to age 41 years and 364 days, you can walk into a recruitment center and sign up now.
Handling basic? If you went out to the general public (male and female) and asked a 39-year old about their physical shape (to handle basic training)....I'd say fewer than 10-percent are in that type of physical shape.
What older guy (or gal) would walk in and enlist at age 38 or 40? Maybe if you were seriously unemployed (ex-teacher burned out, for example)....it'd make sense.
The chief issue I'd see....you'd be in four years....an E-5 (Staff Sergeant). On life experience, you'd probably start to seriously question both officers and enlisted above you.
As for numbers of people jumping in? I'm guessing there might be a hundred folks a year who fit into this older enlistment deal.