According to the 2020 Pentagon Qualified Military Available Study. 77% of American 17 to 24 are not qualified, of that 77%, 11% are overweight beyond a waiver, 8% cannot due to alcohol or drug abuse, 7% cannot due to mental or physical health, aptitude or conduct was 2%, and multiple reasons was listed at 20% including a combination of the above and factors like prior convictions.

So low end 48% of 17 to 24 year olds are inelligble. This doesn’t include specific draft exemptions like being in college or working with critical infrastructure which have always been exempt from selective service.