adding on,
piercings are fine when a child knows how to look after them and only you can know when that is. however (not unpopular), you shouldn't pierce a baby's ears just because you want them done, as i said before, children are not an extension of you, they are their own person.
i got my ears pierced when i was 8 because i broke my ankle and it was the one thing i really wanted done. at that age, i was mature enough to look after them myself.
however, my friend also got them done at 8, and hers got infected because she couldn't properly deal with them.
likewise, i got a tongue piercing when i turned 13. i was completely able to look after this myself so my parents didn't mind. they might not think it looks good but i liked it so they went along with it because they knew i was responsible.
tattoos don't have to be sentimental nor do i think they should strictly only be done once you're over 18. i did my first tattoo when i was 14, and even though i think i could have done better and don't necessarily enjoy the things i did then, it is nice to look back on them, and treat my body like a sketchbook. i will probably end up covered in tattoos by the time i'm 90, because i add them as i like something new.