I think parents point was that being 18 doesn't demonstrate said capacity either. Some kids are mature enough to make important decisions at 15, some kids can't make a good decision if their life depends on it at 25, so the blanket statements one way or another are obviously bad.

We chose an age that made sense to us, because the law needs an objective anchor point, not a philosophical one. However, just because the age makes sense to us legally and socially, doesn't mean it's enshrined in natural law, and we shouldn't necessarily view it as such.

I don't think this is as clear-cut as you write. I'm not sure what the law is in other countries, but in Poland, minors can make transactions, i.e., purchase stuff. This assumes that minors can consent to certain things. The burden of making sure that the transaction is fair is on the adult, and the type and value of the transaction must be adequate for the minor's age. 7yo can buy bubble gum, 12yo can purchase a book, and 17yo can purchase $60+ game. However, if an adult sells $400 laptop to 12yo, the parents of that 12yo can demand the seller to return the money to them without returning the product (ideally, they would return it), or return damaged product. The rationale is that 12yo cannot correctly handle such a valuable item, and the seller should've known better. I'm pretty sure the 12yo would not even need a receipt for the laptop - after all, the seller might not have given it to them (of course, some kind of proof would be needed).

I assume similar laws exist in other countries, and hence societies recognize the capacity of minors to consent to some things.

So all of a sudden a switch flips on in the brain when you turn 18 that allows one to consent?

A 12 year old requesting orange juice does not require a legislative act. Thus it clearly falls into practical capacity for decisions.

To argue otherwise would be odd. Perhaps you have misparsed my comment.