I think quite a few lied about their age to go to WWII. My father-in-law was in combat in Europe on his 17th birthday, so with training- even if it was short training he had to be 15-16 when he enlisted. I don’t think it was considered a big deal then, they just needed so many and most considered teen age boys to be men then anyway since most did a man’s day of work.
William Fairebairn lied about his age and joined the British Army at age 12. He eventually became head of the Shanghai Riot Police. I have read that he was considered the toughest man in the army. He learned Jiu Jitsu from a Chinese master and was the first non Asian to become a master himself.
He taught the British Commandos and eventually the American OSS which became the CIA. He never drank, smoked or used profanity in any way.
Keep in mind, too, the average eighth grader had a better education than most high school kids now, and did their math without a calculator.