Practically everything should be considered in moderation. There are very few, if any, absolutes. It comes down to where we draw the line. Should people be allowed to smoke cigarettes and drink? – Yeah, at a certain age. What age do we set? Okay, how about marijuana - only for medicinal purposes. Should the very sick be allowed cocaine? How fast should people be allowed to drive – 40 mph, 50, 60, 70, 80? Should people be allowed to gambol, engage in prostitution, incest, bestiality, etc? Should people be allowed to skydive in state parks or drive motorcycles without wearing helmets?
At 18 we consider them adults. They can enter into a contract, go pick up a gun, be tried in a court as an adult, fight for the nation and vote. It seems to me based on this, its a good age to set as "competent to make our own decisions".
Weed should be legal for recreational uses just as alcohol and cigarettes are. No rationale for those two to be legal but weed to be illegal. The sick should be allowed ANY drug or substance that will HELP their pain or their condition...purely a medical decision and viewpoint.
Prostitution should be legal, consenting adults and all that.
Incest? If they are consenting adults possibly, I think the hitch here comes under the potential baby and the obvious health problems that can create so on that basis, I think they can keep aspects illegal. Most incest is RAPE and forcible, thats not legal nor a personal choice without a victim.
Beastiality should remain illegal because it harms a living creature that is not in a position to give consent...much like a child is not in a position to give consent.
Gambling should be legal, but games should be fair and consumers should be protected from SCAMS.
Driving speeds are not just about the driver, there are other people on the road you endanger. Driving speed limits should be based on safety, which are determined by road conditions, location etc.
Motorcyles without helmets, they should be able to since they arent risking someone elses head nor anyone else on the road over it.
No skydiving where you can land on people and kill them, again this is a safety issue for OTHERS. If people want to engage in dangerous activities, they should be free to do so but they cant do them in a way that endangers unwilling particpants.