Really good timeline! I was just gonna add that I read somewhere (in Viz's volumes, I believe) that every ten years, a trainer from Viridian Forest is born with special powers. Since Lance doesn't look older than 25 in the Yellow saga and is clearly older than Yellow, this would mean he would've been born ten years before Yellow, making him 20 (RBG), 22 (Yellow), 23 (GSC), etc. I feel this fits his appearance as well. However, I want to check if the Japanese volumes also stated the ten-year thing, since if it's only Viz's who did, it's technically not canon. I'll check my Japanese volumes and get back to you on it.
EDIT: Yes, it's in the Japanese version, too. At the end of chapter 294 (in the FR&LG saga), there's a summary of Yellow that states there being one child born with special powers in Viridian Forest every ten years. I don't know what everyone else's thoughts are, but that's confirmation enough for me regarding Lance's age.