Question for mums? please?

Okkk so I'm 15 and have wanted to get my nose Perseid for a few years at least now BUT no matter how I ask my mum she says "no not untill you are 18" but when I asked her for a tattoo she said yes right out. I even showed her which stud I wan'ted its the smallest one you can get and she still sid no I asked why and she said she didn't wan my puting holse in my boddy (exsept for my ears) now myself and all the people I have talked to about even my best friends mum (who is good friends with my mum also) said the rather have a tiny hole that will grow out rather then a tattoo which lasts forever. my question is. Does anyone know what she means? and why she is letting my get a tattoo but not a little dimond nose stud that she said would look nice on me anyway?