That's nonsensical and blends legal consent and practical capacity for decisions. Most 12 year olds can tell you if they like orange juice with breakfast when asked if they want some. An affirmation is a consent.
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.
15 year old brains are not fully developed. No 15 year old, however brilliant, has the capacity to consent. Though I agree 18 is an arbitrary cut off and it should be higher.
Edit: Below I am replying to the reply to this comment here because apparently 5 posts in an hour and a half is enough to trigger rate limiting for me.
Yes, that is too young to make that decision. You can throw any hypothetical, emotional situation at this and the answer will be the same.
All 15 year olds have the capacity to consent depending on the consequences of what they are consenting to. Should they be trusted to make life and death decisions for themselves? No. Should they be trusted to decide their lunch food? Mostly, yes.
Do I think some 15 year olds are perfectly capable of deciding to work? yes. However, I think it's prudent for us to ignore those few and assume not, simply because there is no universal test for competence, and the vast majority are not.
Imagine you are 15 and your parents are struggling and you getting a full time job in the summer and part time job in during school would keep your younger siblings fed. That's "too young" to make that decision?
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.
The 18th birthday is not a magical doorway.