That might be it. I'm 29 (and a half; wizardry licence soon lads), and by the time I was in year 5/6 (2002/2003) we were all infatuated with "adult" stuff like GTA, Austin Powers Goldmember, Bo' Selecta, and 8 Mile, and the nerds were more into stuff like LotR and Warhammer. You were considered "gay" if you liked kids cartoons at that point, so I'm guessing that DBZ must have come over around then or slightly later.
I remember that Spongebob, Beyblade, Pokemon after Gen 2, and the Gamecube were also things that were considered "only for younger kids". You might as well have said you wore nappies if you were in my year and admitted to liking any of that stuff.