I'm 27 and I love catching it on Cartoon Network (here, at 5:30 in the morning). Granted, I watch it with my 9 year old while he waits for the school bus.
Since its anime, i would say teens and early adults watch it because its the appropriate age for having such story to understand and being it as a cartoon animated way. 12 - 20 i guess, but not anymore now, i guess.