Odd homoerotic context
By making the main character 18 years old, the memory bonding scenes were imbued with an odd homoerotic context. If actual memories of sex and sensuality were shown being distributed from the Giver to the Receiver, I think it would have actually been awkward to watch.
That would have been a gutsy choice to make by the director, but I doubt that was the intention.
If he been an actual child, however, as in the books, I don't think that would have crossed any but the most prurient minds. The Giver's memories of human sensuality would have been met with a prepubescent brain, not a repressed young adult one.