That happened to me over the weekend.
My tama died a few days and of course another egg hatched. It turned into Kinakomotchi. I thought that tama was only available to second generation and on. Then....i neglected it leaving it with no hearts and it still turned into a young mimitchi. :)
Im in the same situation as you. My girlfriend's tama and mine are in love ( 4 hearts) Both are on the Adult stage expect mine is 7 years old and keeps getting bugged by the matchmaker, here tama is 4 years old.