Basically, I will make decisions to do things without my kids' consent if I deem it necessary for them - health, wellbeing, etc. So submitting to doctor's examinations, or having to wear a seat belt, etc, etc. If they want to make alterations to their body - and assuming it won't harm them (or anyone else) - then that's really up to them. It's their body. I think there is harm - however minimal - with piercings, which is why I'd prefer they were a bit older before making that decision.

Re clothes and hair - these things have no appreciable impact on myself or others, so I won't impose myself on my kids in these areas*. If it's freezing and they refuse a jumper, I'll pack one anyway just so I can be smugmumwhoknowsbest.

*Ok, I admit it, I pester my son about wearing underpants.