That’s because you’re looking at this ass backwards. The law determines an age at which a person is considered capable of making legally binding agreements, like providing consent. In the UK that’s 18, with some exceptions.
It would be absurd for the UK to create a special exception to allow underage youths to consent to user tracking at an age lower than the standard age for legal consent in any other context.
its more complex than that. There are many agreements you can make younger than that. Kids can, for example, buy services such as transport (e.g. the monthly bus tickets my daughter bought at 18) that are subject to legally binding agreements. They can buy things in general, on and offline. You can enter into employment contracts in general at 16, and for some limited exceptions even younger.
It would be absurd for the UK to create a special exception to allow underage youths to consent to user tracking at an age lower than the standard age for legal consent in any other context.