Many if not most countries state the general age of the ability to do you your own decisions with the overview of the subsequent outcomes of these decisions with the age of 18. And there lies the problem.

Generally people under 18 years (and the people above aren't excluded) aren't estimated to fell such decisions. Thats why they aren't of full age in the legal context. So while there may are exceptions, people who have more off such an understanding of their decisions and goals (so mostly the full aged people with life experience) could take advantage of the younger ones, in this case sexually. And that is a no go.

While we could argue if a person of 20 years should go to jail when having an affair with a 16 year old (we could argue for hours, where the ethic limit in this cases lies, thankfully law does that for us) my opinion it is non acceptable for someone twice the victims (yes, in these cases they are victims) age to have contact with this person in a sexually way; because the victim is most likely not able to understand all outcomes of this decision.

So yeah, if you say 15 years old should been able to have sex with 40 years old, you are wrong and an asshole, because you want to force other people to share your perception and limiting these age of consent is a thread which can support child abuse.