Kedusha wrote on 10 Dec 2009 23:04:
With my sons, the issue of Baal Keri came up well before puberty, when we were learning Mishnayos Berachos. They wanted to know what the term meant (their Rebbe didn't explain it, other than to say such a person was Tamei). I explained that it's hard to understand at their age, but that when a boy reaches about age 12-13, it becomes possible for him to become Tamei in this manner, due to an emission (not my exact words). When they asked how it can happen, I explained that it can occur in their sleep, or it can happen by touching the Bris Milah, which, I explained, is not permitted.
Even before that, they knew (because I told them, when it came up) that it's Ossur to look at women who are dressed immodestly. So it was perfectly natural that, when they were learning Mishnayos Berachos, I explained to them that looking at such things is another thing that can cause a person to become a Baal Keri.
Because they were not yet facing the Nisayon, the conversation was relatively relaxed from both sides. Well, at least until they would ask if it ever happened to me!
Once they become of age, I think this type of conversation becomes much more difficult. That's why most men don't ever have it with their sons.
As to how babies are born only after marriage, or how a child can look like his father, one can give an accurate answer without sharing details that the child doesn't yet need to know. I explained that when a man and woman live together as husbands and wives do, such as (sometimes) sleeping in the same bed, this makes it possible for them to have children. There is no need to get into further details at a young age.
When one child asked what's the whole purpose of being able to become a Baal Keri, I explained, without further detail, that it makes it possible to have children. Interestingly, the child didn't want to hear any more!