Start them young is what I say. I bought my Godson a Hornby Thomas set when he was three, and since then he has gone on liking all things to do with railways. Here I am showing him how to make track.
He was quite attentive to what I was doing.
I even got him gluing the chairs to the sleepers on the small bit of track that we made together, and then he got to take it home with him, and I understand it is one of his proudest possessions.