Luke is 10, but he's been an off-and-on part of our gaming groups since he was about 7(ish). We started him on D20, but that was mainly because that was the only game in town, no one wanted to play anything else. I did get my groups to try new things on occasion, so he's played some white wolf, some rifts, and some traveller as well. (Oh, and we did have a rather successful SilCore game based on WWII...with mecha. I used Gear Krieg as a launching point, and then several sessions later, we switched it to Alternity.)
As far as solo adventures go, I plan on winging things a lot for the time being, as this will be the first full-on DMing I've ever done in RM (despite having the books for ages). So single creatures, I plan on making a few NPC's available when he needs them (a healer, for certain, plus various sky-sailors, monks, and so forth). Once we both get the rules tucked well under our belts, I'll start experimenting with multiple creatures, larger creatures, stuff like that. Besides by then he'll have leveled up too.
Now if only I didn't live in the middle of gamer-nowhere...I'd love to open this up to more players.