You said it: as stupid as their choices may be,-which is why we have consent laws.
If letting a 16 year old choose to have sex with a 40 year old and it be perfectly legal, then who's to say letting a 15,14,13,12,11 year old make the same same choices not going to be no big deal to everyone?
If they start lowering the age at which teens can make their own choices and have the law behind them then is the law also going to let the parents legally kick them out of the house and not provide any financial support?
Can't have your cake and eat it too, if 16 year olds should be able to choose to have sex with 40 year olds then parents should also have the choice to to let them make adult decisions like getting their own place, own car, and bills.