Ever since it came out in 1998. I was 5 (almost 6) years old at the time and it stuck with me ever since. I even had some of the merch and themed school supplies! (I should qualify for a seniors discount!)
Though I passed on the 2016 reboot since I was naturally skeptical about them given some of the horrible reboots of other shows that came before it, and I'm glad I didn't bother with it given how they screwed THAT one up so bad that even I didn't want to include it at all in my Powerpuff Girls Repowered AU (my personal revision of the OG '98 series). I could say the same for the CW's live action reboot, but that one was canceled so I cannot officially count that one despite the leaked script and trailer (that even WBD is trying to erase out of existence).
Though I wasn't really into any of the comics at all, seeing the tv show format as the proper/dominant iterations.