I don't think there are any restrictions based on age. You probably read something related to the incentives or credits given by some municipalities to young families with children to encourage them to settle in their town. In those cases, yes, I would understand that the families need to meet certain criteria to qualify for the benefits.

I saw that some incentive in Ibaraki was till age 43. They give you money to relocate if you're working in Tokyo. I don't think you could possibly qualify for this specific incentive as a foreigner however restoration and some others might work.

Can I buy it for my granddaughters or grandson?