Well I agree with your point of a 18 year old, especially a young man, without your conscent getting one in a not so sanitary situation. And also as long as it can be covered for working etc, that would be fine with me. I will admit a tattoo on a woman gives me a quick eyebrow raise, but see nothing wrong with it for a man. My step son, the ex marine has two. Very tasteful, and mostly able to be covered. ( on his arm, and his chest.) Please remember- at 18 they are able to be drafted so they are adults. They can also vote- he does not need your consent, and all we can offer is opinion..
My daughter is now 21. All I can do is advise, and I have found being supportive not only has her more honest with me about adult problems to "discuss" with "mom"- but also having her trust, I know more what is going on with her. ( she lives 60 miles away.) I would have to say yes, and offer to help him find a good tattoist. Remind him he will have this for life, so it needs to be done WELL... And where ever he puts it, if the muscle mass behind might change with age, he needs to consider that as well. I think the upper arm is pretty popular..