I just thought of this...
If Son of Godzilla was released and set in 1967/1968, and that was the year Minya was born, so he was not even a year old. So, thats straightened out...
UNTIL...
Destroy All Monsters was made in 68, but takes place in 1999...so how come 31 years after Minya was born...he still looks the same?!?!