Boys can have erections from before they are born. It has been seen in many fetal ultrasound pictures. They don’t generally have orgasms or ejaculate until adolescence. Most children don’t have much knowledge of or interest in sex until their bodies start to mature. This innocence should probably be maintained as much as is reasonable, because anything that’s psychologically troubling for a child can have effects that last a lifetime.
That’s why our society frowns heavily on pedophilia, sexual interest in children. What may seem like a simple act of kissing or fondling, or even just discussing sexual concepts in the wrong way, can literally damage a child for life. The mental processes that ensue can confuse kids tremendously, resulting in assumptions or conclusions formed in not-fully developed brains that stick in their unconscious minds forever, destroying their self-confidence, and ability to make rational decisions. For instance, a child who’s body has reacted with a pleasurable response to a sexual act will feel that s/he is to blame, is a horrible person, and must work very hard to keep secrets.