The Army estimates that less than 27% of American would qualify for enlistment according to standard enlistment criteria:

• Ages between 17 and 34 years old
• Must be U.S. citizen or legal foreign national
• Must have high school diploma or equivalent
• Minimum 33 score on Armed Forces Qualification Test
• No tattoos on fingers, neck or face
• No ear gauges
• No ADHD medication win the past 12 months
• No felony convictions
• No persistent illegal drug use
• No insulin-dependent diabetes
• Meet height & weight requirements

The major reason? Young people are overweight and others are subject to mental-related restrictions. (I am disqualified due only to age, and meet the weight requirements for any age.) We also have a report from a doctor that 1/3 of Americans under age 19 are either overweight or obese. (Probably because their toys require electricity instead of sweat.)