They're not able to sign contracts or take out loans. But sure, they're mentally mature enough to decide to have life-altering body surgery.
That's the wrong angle to take when trying to argue "X shouldn't be legal until you're 18!" We let kids do much more dangerous shit than signing contracts or taking out loans before they turn 18.
We pretty much universally grant 16 year olds the legal privilege of operating a motor vehicle without supervision. You can do a lot more than mess up your own body with a 3,000+ pound mass of plastic, metal and glass on wheels with 150+ horsepower at your command. You can kill (lots of) people, cause immense property damage and destruction, disrupt the normal activities of thousands of people if you cause an accident, etc. And it can happen in an instant, as the result of a split second of poor decision making, anger, panic or simple inexperience.
We let them take jobs at 16 with few (if any) restrictions beyond max hours per day/week, and even younger than that with modest restrictions on venue (no steel mills) and work tasks (hands off the forklift). We don't even particularly care about kids barely entering their teens working on (real) farms handling animals and equipment many times their size and strength.
Then there's sex, which we (collectively) never want our kids to ever do because no boy/girl is ever good enough for our son/daughter, but we all know they're gonna do it whether we call it "illegal" or not and so we just slap a number on it (16 or 18 depending on region) and hope for the best. Raising the age of consent to 18, teaching abstinence-only and hounding 17 year-olds for holding hands didn't reduce the teen pregnancy rate -- it raised it (and it made the nation's degenerates insufferably smug in the process). Nothing makes a rebellious teenager want to do something more than telling them they can't.
Are people under 18 mentally mature enough for, well, any of this shit? Nah. But neither are 18 year-olds (contracts, loans, smoking, most adult-only activities besides drinking and gambling). Neither are 21 year-olds (drinking and gambling). Or 25 year-olds (lower insurance rates, car rentals). Or even fucking 50 year-olds (finally permitted to buy nice cars without "mid-life crisis" accusations). Show me a 55 year-old man who doesn't giggle at
Forklift Driver Klaus and I'll show you a man you should
never play poker with.
The training wheels have to come off at some point, and people eventually have to start learning through their own experiences and reasoning. Yeah, people are gonna get hurt here and there, but for the most part it all works out okay. Coddling people forever by excusing their dumb decisions because of their age produces weak, dumb people who make dumb choices and hurt themselves more in the long run.
Blame the parents, therapists, doctors, groomers and the internet in general for teenage trooning if you want, but I have to agree with the sentiment that the buck stops with the patient. If you lack the tiny bit of self-preservation instinct it takes to realize "no, slicing off my breasts/penis/testicles/whatever is
not going to feel good and I should not do this," then perhaps your sole purpose in life is to serve as an example to others.
I know someone who considers herself bi, but also essentially straight and not part of the LGB because all of her relationships have been with men.
Then you know a straight woman who's trying to make herself seem more interesting by pretending superficially to be attracted to women.
Bisexuals piss everyone off (on both sides of the fence) because they're the vegans of the bedroom. They're even pickier than gays and always make sure to let everyone know they're bi, but "just aren't into you." But they're totally bi though. Did they mention they're bi yet? Because they're bi.