Ahh okay, I've read some wrong information then..
We just wait until age 7 and we had no problem of getting ours to mate at that age =)
Actually you were partly right, Mimitchi.
Oto and Oji are not compatible with adult stage characters, only with each other (= oldies), and there appears Oyajitchi if you get to mate them.
I think this Oyajitchi things is joke which understanding is easier if you are common with japanese society.
Like, Oto and Oji are old couple whose only son (Oyajitchi) doesn't have wife or kid(s), he has only his work and that's all his life.
He even has cafein-ism after drinking so much coffee to stay awake with all his extra work in late hours