in your post, you make a good point....though someone could be 21, they may have the mindset of a 2 yr old, etc...I knew a family next to a cousin of mine...one son was "normal", the other was MR and had some understanding of right/wrong, but could not put impulses and actions together...he caould not make a logical and informed thought of "hey, thats wrong, therfore I should not do it", also, he did not understands consequence vs actions,etc...
for instance, in his early teens, he started to be hyper=sɛҳuąƖ and could not amke informed decisions about touching people...sadly, he at times would publicly mastrubate, yet when caughter, could not put it together that it was wrong, should not be done in public,t all anywehre and that there were consequences-socially and morally....
he is in late 30's or so, stil llives with parents...they have to keep eye on him as a neighbor where they now live as a daughter that is MR and same age, they have to watch he does not act inappropriately with her....again, not talking normal urges and choosing to act on them, but he has a body of a man, mind of someone likely around age 5 or less...
should sex happen, likely would not be sinful or carry the weight that thos of us adult "normal" people would incur..
again, need a good confessor to assist and counsel