At least three, as far as I can tell. Probably more.

“Probably”? I think the only advertisement I’ve ever seen from Valve was for Half Life: Alyx when it came out a few years ago. Not exactly brainwashing kids

Additionally, kids are not the largest group on steam. According to Statistica’s Consumer Insights Global Survey, the majority of users are 30-39, with the next largest group being 20-29 https://www.statista.com/forecasts/1242344/steam-us-user-share-by-age