Informed consent and legal ability to make certain decisions are different concepts. And in the cases you list the key difference is giving consent for a medical procedure which may have drawbacks but which may have upsides too, and asking the patient to weigh those aspects before deciding. Whereas the things you listed that a teenager cannot do are generally not seen as having a benefit to the teen.

Informed consent is more or less exactly what it sounds like. It’s the idea that a thing is explained to you in a manner you can understand and then you agree to it. It’s a thing that can be given even by very young children for example. It’s also a thing that an adult may not be able to give, for example if they have certain developmental disabilities. A large chunk of medicine and medical research is built around the idea of informed consent. The idea that the patient needs to have medical procedures explained to them in a manner they can understand before they agree so that they can make their own judgements about if the risk is worth the potential gain.

The other things you mention have to do with legality, which is a policy rather than scientific construct. For the examples you listed the idea is that those are things we generally don’t want children doing because they are 1) addictive and 2) bad for you. The age cutoff is the age at which we as a society will let you make your own self-destructive choices.