It's illegal to claim they consented because it's impossible by the current societal understanding of what it means to consent for them to have consented. A 14 year old can understand what they want, which used to be good enough to be considered consent, but now that we know their brains aren't finished developing yet, we don't consider their ability to understand the repercussions of the decision to be on the same level as someone who's fully an adult, and thus they are incapable of consent.

It's like how a 2 year old will want candy all the time, but you shouldn't let them have it every time they want it because their brains aren't developed enough for them to fully understand that it's not good for them. You have to be capable of understanding the long-term implications of eating nothing but candy, and a 2 year old literally can't do that. They might know it causes tooth decay and weight problems, but they don't know exactly what that means because they lack the brain power to process the amount of background knowledge they would need to understand it.

On the same note, a 14 year old might want to have sex, but they don't understand all of the implications of it, even if they think they do. An 18 year old might not know the implications either, but we have to make a cutoff, and by 18 we assume enough people should be capable of consenting that we give the right to everybody. At 14, though, it's likely that nobody that age can truly understand it enough to be considered capable of consent by the modern definition, and even if some can, the effort to parse it out just so some rock star can fuck the few that are capable of consenting is obviously not worth the effort.

Just so we're clear, I'm not saying that we need to somehow prevent 14 year olds from having sex with one another - while they still aren't capable of understanding the implications of their actions, we can at least be reasonably certain that they both consent to the same lesser degree as one another. The issue is when one person has the higher mental capacity and the desire to take advantage of the lower capacity of their target to convince them to have sex, otherwise known as statutory rape, the topic of this discussion.