Much depends on the child's birthday. Most states have school cutoff dates in the fall; in Virginia it's Sept 30th. Children normally need to have their birth date before Sept 30, 2021 in order to begin kindergarten in 2026. Those born on or after October 1, 2021 wait until 2027. A child born on 9/30/2021 would graduate HS in May 2039, four months short of 18. Born on 10/1/2021, graduation is in May 2040, eight months after 18th birthday.
In reality, at least where I live, many parents hold their children back a year if they're born in the second half of that window; not all kids are ready to begin school when the rules say they should. My son was born in early July and was almost always the youngest child in his class, and occasionally the youngest in his grade. In kindergarten he had a couple of classmates who were 18 months older; they'd flunked kindergarten the first time through.
The youngest college wrestler I can remember hearing about was Cornell's Jerry Rinaldi, who supposedly was still 17 when he first suited up for the varsity. Colleges don't post birthdates anymore, a good idea to protect privacy.