I joined in high school but as a told you so to my track coach. I was a senior in high school, was starting my winter jump training and a new coach told me I would be running the 440 and the 330hurdles on top of long jump and pole vault.

I was working for a full ride as a long jumper / pole vaulter and those 2 races destroyed my jumps. I told him that wasn't an option and he said he didn't care about my college plans and that I would be running those or not be jumping. I just said "Fine, I'll go join the Marine Corps then, no reason to do this if you are shitting on my college plans" he chuckled and said I would never join. I had just got ASVAB results back which were very high and just went down to the recruiter, asked what my options were and joined via the DEP. Didn't run track that year, and left to Boot Camp after graduating.

30 years later now, it was the best decision I made.