You don't stay the same and have the same likes, interests and gases all your life. Many people who get tattoos severely regret it. The you her they are when they get them the more likely they will outgrow and then dislike them.
They look awful when you get old and your skin sags and wrinkles and they can limit your employment prospects.
Tattoos are certainly permanent and I think some people get some pretty ridiculous things inked on their body. I was 26 years old before I finally made the decision and I've stopped with two. I'd like to get more but they are expensive. I think teens need to realize that tattoos are permanent and depending on their location they can affect a lot of things in their life. For this reason I think it is best to stop minors from getting tattoos before the age of 18, even if the parents consent.
I can't even imagine what tattoos I would have if sub-eighteen me was able to get them. Tattoos are permanent and there should be great care in deciding on what to get. Also, there are contractual considerations to consider as any contract made by a minor is automatically not binding. So, any wavers that need to be signed against law suits due to complications would be invalid anyway.
I think it's perfectly ok for a minor under the age of 18 to get a permanent tattoo with their mom or dad's consent - I just think they should be sensible about what kind of tattoo they get that young, because after all, they are permanent. I know teenagers and if you forbid them to get a tattoo, most of them will either have a friend who does tattoos, or they will find somebody that does it out of their basement.
Minors should not be allowed to get tattoos unless they have parental approval. Even then, both the parent and child need full psychological testing. This level of permanence means that both need to be vetted deeply and fully. I would think that a tattoo artist needs to have a counseling license to deal with these situations.