I don't think "most people" have tattoos, but many people do, and all sorts of other body mods -- such as plastic surgery -- as well. Their bodies, their business. Prudence was way off the point with her non-answer, and I agree it reveals her prejudices.
If you don't like a tattoo later, it can be touched up, covered, or removed -- again, your business. I know many older people with tattoos and they look just fine, far as I can see. Maybe it will be an expensive and painful mistake -- and maybe not; the people I know who have them, love them -- but it is not irreparable if it is.
I suspect, as a parent who isn't nuts about body mods, the easiest way for the LW to tell his parents, and he should if they'll see it or hear about it, is to bounce up happily, in the presence of others, and say, casually, but with a huge grin, "Look at this! I got it last week -- isn't it (cool, great, beautiful, whatever)?" If he makes a big ugly deal of it and slouches up all hangdog and shamefaced, he will get an entirely different reaction. Be lighthearted, and give them a chance to get over their shock without blowing up in knee-jerk reactions.
I do agree with those who say anyone contemplating a body mod that is irreversible or very difficult to reverse should make darned sure it really is something they will want to live with forever. It is also possible to get fake tattoos, even custom ones, that last a couple of weeks. With those, one could wear the desired image until making it permanent (or not) becomes inevitable.