Indeed. That actually got me curious as well so I had a deeper look at when each show debuted the first heroes' Perfect.
Adventure - 20 (or 16 if you technically count SkullGreymon).
Zero Two - 26 (obviously I didn't count returning heroes from Adventure going Perfect).
Tamer - 14
Frontier - 10
Savers - 13
Appmon - 19
I didn't bother checking Xros Wars since y'know "no Levels" and whatnot.