the military tracks how people are becoming more fragile? how are they measuring this?

I guess they'd measure injury rates of new recruits.

Have they not considered extending basic training to give peoples bodies more time to get conditioned?

If you start out reasonably fit it takes less time. If you are heavier you need a longer runway.

Maybe a pre-bootcamp for large people where they introduce basic nutrition and basic exercise. That might actually be a big draw for obese people who want to turn their lives around.

Already a thing https://www.military.com/daily-news/2022/11/23/army-expandin...

How much more time? If the problem is lower bone density or fragile joints it’s already too late.

Why should it be too late? You increase bone density by training. If the military finds people have not enough bone density they need to add more fitness exercises (e.g. strength training) to initial training.

What young, obese people are suffering from permanently decreased bone-density? If anything their bones should be strong from moving around the extra weight all the time. These are young people, they should be able to adapt if given enough time.

What are you on about? It's well known that bone density and strength increases with load: https://en.wikipedia.org/wiki/Mechanostat

If you didn't learn this in grade school it might already be too late.

Then you will have selection bias because new army recruits probably are a bad representation of “the youth”

The new recruits are actually a best-case scenario - going more than a decade back, ~75% of potential recruits that wanted to sign up were rejected for being clinically obese. Of the 25% left, a significant portion could not complete the first physical test. The test is one minute each of sit-ups, push-ups, and a one-mile run.

This [0] was linked up-thread and touches on increasing injury rates that the US army has worked to get down

[0] https://invid.xethos.net/watch?v=sWN13pKVp9s

Injuries during basic training, but especially bone related injuries that are very expensive to deal with.