The problem is that there’s a huge power differential when one member of a relationship is much older. While there may be individual exceptions, in the majority of cases it’s going to be hard for a teenager in a relationship with an adult to avoid being exploited. A relatively inexperienced teenager is going to have trouble recognizing warning signs, setting limits, expressing needs, etc., with the result that the older party may end up taking advantage of him/her even if the relationship starts with the best of intentions.
Power differentials are of course just as possible and potentially problematic in adult relationships, but there’s a point at which people are expected to be mature enough to take responsibility for any problems. Our culture generally sets that cutoff at eighteen, and though that’s somewhat arbitrary and doesn’t account for individual differences in maturity, it makes sense as a legal construction and affords some degree of protection to people whose brains are still developing and chemically predisposed to compromised judgment.