> > How old do you think is old enough?
>
> I don't like to put a specific year on things like this because everyone is different.

You've hit the nail right on the head with that statement. Every person is different; each child matures at a different pace. Some children are very bright, learn quickly, and would be capable of understanding basic salvation doctrine at an early age. Others take longer. And of course many become mature enough to understand, but choose not to accept it until later in life, or perhaps never.