Legally speaking, NO. My question is, why would an 18 year old girl want to have sex with a 10 year old boy. (I would ask the same question if it was reversed and it was a 10 year old girl wanting to have sex with an 18 year old boy.
I can understand how a 10 year old boy may become infatuated with an older girl (same if the ages were reversed) however, at that age and that age difference there is no "consensual" sex. The girl in this case could be charged with statutory rape and/or child molestation. Now understand, the parents, nor the 10 year old, have to press charges, at that age and with that much of an age difference (the 18 year old being an adult) the State or D.A. can press charges with out consent of said child's parents.
Now if you are asking if the 10 year old can physically have sex with this girl, it is possible to do something close and yes what would be considered sex. At that age a boy is able to get an erection and penetration is possible, however, the boy is still developing and growing, his hormones are going to be kicking in and reaching a climax during sex at that age is not the same as an older teen or adult.
So to answer this question....
Physically, yes I believe it possible.
Legally and Morally, NO.