I have been into miniatures and Warhammer & 40k for many years already. Guess when I started this hobby I don’t even have a son yet then. Over the years of painting & gaming and son growing up, my son sort of got interested in what I am doing.
When he was about 3 years plus, he was already helping me base coat black my miniatures. As he got into the 4 years old range, I took some of my old bretonnia archers miniatures and let him paint. Now he is 5 years old, I got him some basic Imperial Guards Cadian snap on miniatures. Help him assemble up and let him paint all by himself. He wanted to paint like the box color scheme, ready the color for him and he paints by himself. Help him put the white glue on the base, and he based it with sand and paint when its dried. Now he has about 10 Cadian Guards, 5 more to build and paint, going on to have 15 guards.
Occasionally, he pestered me to play with him Warhammer 40k as well. We played with simple rules, 2 squad of 5 men running around shooting and assualt. I was surprised at one time, he got the idea of using his 2 squads of 5 men shooting at my 5 men squad which will have a higher chance of hitting and wounding. With a few games more, he got the idea of shooting at 1 squad and using another squad to assualt! Cool isnt it, for a 5 year old. Well, its a Imperial Guardman in the training! FOR THE EMPEROR!