Thats a tangled issue, I am pretty sure the law states that whilst under the influence of alcohol you actually are not responsible for your actions. (mainly referring to minor indescretions but this is always overlooked as the fact you allowed yourself to get into such a state of intoxication in the first place. But even then you are under the influence
) . I am fairly certain this has been used in drink driving cases.
How ever as you rightly pointed out in this scenario the adult would be hounded by the police and placed on the sex offenders register. The youngster would be told off.