Had no clue too about the difference between that year and our year.
No planet ever mentioned other than Planet Harriers and Planet Menon, but that's not where they lived.
Their planets were just always refered as "homeland".
I don't think they're manufactured.
They could have babies that look like these ...
Also there are children too..
The ending for Fantasy Zone Gear features several Opa-Opas (Opa-Opa’s children?) rescuing Opa Jr :
which means their body could grow up, and we know how big O-papa is in the first game's final battle.