So one of the very first tabletops I ever ran was just for me and a handful of friends, and my at-the-time boyfriend. Well, kid sister to one of the friends was like 12, and wanted to get involved in some of what we were doing. Kid sister was pretty cool, so I agreed to let her play.
This kid, man. She soaked up pretty much the entire rulebook inside of a day, was asking me advanced questions about gameplay by the end of the week. Hadn't even played yet, but I was starting to get a feel for where her mind was going.
Well, I knew that the others at the table were either going to take it easy on her for being new, or try to do the whole newbie initiation hazing thing that they do, and I was starting to get a feel for how her mind worked. So I set her up as one of the villains of the story, infiltrating the group and actively working to undermine them from the inside.
And man, this kid sold it. Every time they would encounter a setback that she had cleverly engineered, she would be the first up in arms, calling for them to mount up and ride off, no time to wait or rest or prepare, we have to go get that son of a gun!. And the rest of the people at the table are just like "Yeah, we can do it, rar!"
And I'm trying so hard not to just lose it because this kid's acting is worthy of Hollywood, right? And she's coming up with just the most devious stuff -- we had agreed that if she had a question she could write it on an index card and I would write the answer back, since she was new and playing shy (this was well before texting was anything like easy or affordable, SMASH THAT BUTTON 4 TIMES FOR S, 25 CENTS A MESSAGE), so nobody else was catching on.
So by the end of it she's managed to sabotage half their efforts and prevented them from resting in their haste to race to the bad guy, and when they show up, she just casually adopts this big, chipper smile like 'Alright, they're all yours now, I'll be in my room!' Grabs a blow-pop from the big bowl, and just straight up walks out of the room with a little skip in her step before the big boss battle.
They still managed to win, but man, it was a tough fight, and they never underestimated this kid again. She was a regular in our group for like four years, and is easily one of the best players I'd ever met. I'm thrilled that I was able to introduce her to the hobby, and I hope that she's carried that torch forward.