Wraith: By far Quest for Glory VGA. My dad is a huge fantasy buff but also likes comedy (Myth Adventures for example) so Quest for Glory fit him perfectly. Around the time when Windows 3.1 (I think) came out, he gave me his old DOS PC that auto-booted to some menu that had a list of all installed games. Just press the number and play. Quest for Glory VGA was on it, and I spent every single summer replaying through the game or the series when we got the sequels. Sure, there were other games on their (Win Lose or Draw, Bouncing Babies, Test Drive, Disney's Matterhorn Mountain, Pharaoh's Tomb, Battle for Middle Earth, and a few others I can't remember.)
A runner up would be Leisure Suit Larry. Going through a box of those big floppy disks (Not the standard floppy disks we have today) I found a copy of Leisure Suit Larry. It had the Sierra logo on it so I figured it would be a fun adventure game. My dad knew I had installed it but I don't think he figured I would understand it or get far in it. Since I didn't know many of the words, I wrote them down. You can imagine my moms outrage when she found a piece of paper with the words "buy prophylactic" written in blue highlighter.
How old was I playing these? Probably around 9-10 years old. I played QFG and LSL games on the DOS machine until around 94 or 95, then I had to migrate to using my dads PC since the DOS machine couldn't run games like QFG 4, Kings Quest VI, or Police Quest IV and SWAT.
EDIT: AHA! I knew I forgot one! The very first PC game I played was Hugo's House of Horrors when it was first released back in 1990. I was 7 or so.
yeah the good old quests games where you could die and die a lot :) and off course a lot of discs. it was rgeat when the voice edition came wich was expansive that day.