Why there is a minimum age limit in 40k
Yesterday, I ended up realising why there is a recommended minimum age limit at my local gaming club of 12+. I found this out when trying to play in a 40k game with 3 young children as my opponents against my CSM's. The game only got three turns before time ran out because they ended up running off and playing about quite a bit of the time, not playing the game. This activity also ended up annoying players in other games, as their actions were disruptive as well.
Basically, there was the impression that children whose ages are at the single-digit level may be quite happy to paint their plastic/ metal models but they're not so interested or patient enough to actually sit down and actually play the game.
So, my opinion is that while these young ones may be the 40k/ Warhammer gamers of the future, it is probably best that they wait several years (i.e. until they reach High School age) before they actually start playing. By that point, they should be more mature, able to understand the rules and have the patience needed to play. Until then, they are perfectly welcome to spectate.