The minimum standard already varies based on age. A 25 year old doesn't have to conform to the same standards as an 18 year old. A 31 year old doesn't have to conform to the same standards as a 25 year old, etc.
The PT test exists not because its the best or because it shows combat readiness. It exists because it is easy to administer, easy to score, easy to record, and that is all favorable to a giant bureaucracy like the military. It is, in many ways, arbitrary. Passing the PT test does not mean you can effectively do your job, and neither does not passing the PT test. When I was in, there were plenty of medics who passed their PT test but couldn't deadlift my 220lb ass out of a tank hatch. If I was hurt in my tank, they were useless. I don't give a shit if some 145 lb person can do 12 pullups, they probably can't haul me where I need to go if I'm critically injured. Nor could they carry as much ammo as me, nor could they hump a 80 lb ruck as long as me. You know why? Because I was 220 lbs and pretty nicely muscled through the back and legs, and an extra 80 doesn't matter that much to me, its a smaller percentage of my weight. I wasn't a SAW gunner because I was the highest PT score, I was the SAW gunner because I was the biggest motherfucker in the squad.
If they really wanted to, they would implement job specific PT tests. Other than simplicity, it makes no sense for a cook to have to have the same upper body strength as a bridge engineer. A spatula doesn't weigh as much as a Bailey Bridge piece.
Its ridiculous to say an 18 year old isn't combat ready because (s)he can't do the same number of some repetitive exercise as "they" say (s)he should, but if that same person was 30 they'd pass. Is combat easier for people who are older? No, its the same fucking combat.
(Now here's where some folks will say, "well, 30 year olds tend to be in leadership or admin positions, so let's stomp those arguments before they start)
1) PT standards are the same for enlisted and officers. Officers are *always* in leadership or admin positions.
2) Enlisted men in leadership positions still are on the tip of the spear. Sergeants and staff sergeants especially are right there with their squads and are as much in the mix as the lowliest private.
3) Not everyone enlists at 18. There are 30 year old newbies.
In short, gym performance is not real world performance, but is used because its simple to administer, grade, and record.