I began with my two grandsons on my knee watching youtube steam locomotive videos. They couldn't get enough. Parents bought them a used Thomas table and a lot of 'stuff', and they played with it for hours. I also had a working layout when they were upwards of three years of age, so that helped. At present they're across the country, so their current interest is beyond me. They've both started school. I'm pretty confident that if they were on the left coast with us they'd be right into it.
"One difference between pessimists and optimists is that while pessimists are more often right, optimists have far more fun."