Some broad issues this touches upon:
1.) Since this is most likely in regards to pedophilia, the primary reason (from a legal perspective, in most jurisdictions) is because minors, particularly those who do not have at least a semi-independent financial situation and some adult life experience, are usually rather vulnerable to manipulation and abuse, including of the sexual sort. They may not have a good grasp of the risks and consequences. This goes double for younger minors who may have only a vague idea what sex is, if they know at all. While being an adult is no guarantee of any of those things, a clear cutoff age is provably effective at harm reduction.
2.) The secondary reason (again in most jurisdictions, though it can also be the primary one) is that pedophilia can interfere with parental obligation to protect their children from making harmful choices, and from other adults making decisions on the child’s behalf which are culturally the province of either the parent or the child. And occasionally, it can prevent parents from forcing their children to make harmful “choices”. Some jurisdictions do put way too much emphasis on parent’s rights and powers over their own children, but that’s a separate issue.
3.) Most people also find it disgusting, and that’s not a good reason (in itself) to ban anything - but that disgust is also not relevant to the supposed “rights” of pedophiles. Just because some people feel something is wrong doesn’t mean that this is necessarily the *only* reason to ban something and people just need to get over it.
More generally, not limited to pedophilia (or even sex):
4.) If you don’t agree with rules or laws which most people willingly follow, and that you are an exception which they shouldn’t apply to, then you are most likely exactly the sort of person who the rules were made for, and you either don’t understand why they exist or are delusional. They’d be pointless if the only people they didn’t stop were the worst offenders. (That being said, bad rules and laws do exist and most people agree to follow them mostly because it doesn’t personally cost them to do so. But in that case, object to the rules themselves rather than insist on being an exception, though I’m going to side-eye you if you knowingly object to most forms of harm reduction.)
5.) Consent is not unilateral. You don’t just get to decide that someone else, deep down, really really wants whatever you desire to do to/with them. Even if you’re right, that doesn’t negate the risk of harm which comes from forcing the issue - and if you’re the kind of person who frequently thinks that about other people, you’re almost certainly also the sort of person who is very rarely right and usually only by accident. And regardless of the legality of any actions taken to force the issue (or lack thereof) you don’t have the right to sympathy and/or a get-out-of-jail card for accidental abuse.
