There are several reasons:
1) your body is far from finished growing, and your image will distort, blur, have the lines blow out, and might not be distinguishable by the time you are grown. Then you would need a cover up tattoo that would be 2 times the size of the original.
2) your brain also has not finished developing, and will not be an adult brain until you are in your early 20's. What this means to you is that your decision making abilities are not fully formed, and that is why teenagers need guidelines from adults, and it is also why they do not understand why they do. Teenagers base their decisions on their peers (who's brains also are not developed), and they misinterpret many things that are emotionally based. Teenagers are not equipped to decipher the human faces expressions in all of the subtle nuances, and this leads to misinterpretations and decisions based on these encounters with others = rebellion based decisions.
The reasons for wanting a tattoo, are often not good, and therefore the images that are chosen are regretted when the person reaches adulthood.
3) it is illegal to tattoo a minor, and anyone who does so is committing a crime. They are also, more than likely exploiting the teen. It can be dangerous for health and wellfare, and some pretty scary things have happened to teens by 'scratchers' doing tattooing in their garages.
4) Teens don't always have the money for a quality tattoo, and will decide to do some very stupid and dangerous things to get one... homemade ideas, and unsanitary conditions.
So, when doing something that is permanent, you should wait and do it right.
So, this is why Be of age and grown and mature. Find a reputable and skilled and licensed tattoo artist. And be able to pay for it, including the tip.
This is serious stuff, not a fad for a teenager, because it is forever.
luck~