All I know at the moment is that adult characters seem to appear only on certain generations, like on the v1-v3. Here's what I think I've worked out so far (may not be correct):
Male- Odd generation:
Mametchi
Shimashimatchi
Kuchipatchi
Tarakotchi
Gozarutchi
Male- Even generation:
Kuromametchi
Togetchi
Dorotchi
Robotchi
Tosakatchi
Female- Odd generation:
Mimitchi
Memetchi
Furawatchi
Sebiretchi
Masukutchi
Female- Even generation:
Chantotchi
Makiko
Onputchi
Meidotchi
Kunoitchi