I'm pro-body modifications over here. In fact, yesterday I just sat (second session of three) for 4.5 hours on my sleeve tattoo (shoulder to elbow, so only a half-sleeve) honoring my son's birth and also my pregnancy losses. It's a beautiful, custom piece, and I'm thrilled about it. Also, I have 4 other tattoos (upper back, just below the elbow on my other arm, and two that are 4 or 5 inches high full circle around both my ankles).
That being said, the one I got when I was 18 is my least favorite, and I don't think I should have gotten one that young, but I did make a good decision about it's location (upper back) and it's always been hidden in any business attire. In fact, when I'm in a business suit, you can't see any of my tattoos. I very consciously planned it that way. Also, I live in a conservative area and am in a conservative profession, but many of my fellow school district employees have piercings and tattoos (eyebrow and nose piercings mostly, and upper arm and ankle tattoos).
The thing about tattoos is that, IMHO, they should be symbolic and also custom. I don't feel that basic tribal tattoos that come out of a book at tattoo shop (ahem, the one on my back from when I was 18) are beautiful or meaningful (with rare exceptions). Also, depending on physical development, 17-18 is still quite young and a person's body (especially males) can still grow, stretching their skin, etc. Also, they HURT a lot.
My parents begged me to get pierced and not tattooed, the idea being that piercings can always be removed. I did have 11 (I think) piercings in my ears, but never wanted to pierce anything else. I personally don't think facial piercings are attractive, but as a pp stated, beauty is quite subjective. Also, for the record, I dyed my super-short hair every color of the rainbow, with permanent hair dye, from blonde to blue to red to black and back again, over and over. My parent's liked blonde the best (not my natural color! ).
I think it's important to consider that a wide variety of cultures throughout history have used body modifications as a right of passage. I personally would argue that teens have a hard time in our culture today because we don't provide them with this type of process. Also there is the psychology of the forbidden fruit that plays a role in wanting a tattoo as well. I never went behind my parents back to get a tattoo, I just waited until I didn't need their permission anymore. ETA: an interesting book to read on this kind of topic is The Quest for Human Beauty by Julian Robinson. It's about the cultural anthropology of beauty. Interestingly, all cultures practice some kind of body modification. He hypothesizes that this separates humans from animals.
If/when it becomes a question with my son, I'd let him have a regular piercing - ear, nose, eyebrow, tongue, lip but not gauged ears; we'd probably negotiate a waiting period for him to gauge if that's really what he wanted. There's no going back with that and it's not hideable by a business suit - (clearly this is one of my sticking points. I love tattoos, but I'm not going to tattoo "RIP Vinnie" in old English lettering on my neck either. Certain criteria apply - anything permanent should be classy, carefully located and done by quality professionals). With tattoos, as I've stated my belief about customized and meaningful work, and also rites of passage, I would probably have lengthy discussions with him about what he wanted to get, what it symbolized, and where to get it. He and DH and I would plan it together, and he could get it when he's 18. I don't approve of under-age tattoos, personally, for the developmental reasons stated above. Tattoos at age 18 are kind of a rite of passage in our culture anyway, and my signing a waiver would take that away from him. Weird, but that's how I feel.