Reason for my younger son to be an adult

I helped my younger son to take bath yesterday (he actually can do it himself but I worried about he would get sick while he played water during his bathing time), the following is the conversation between us:

My younger son asked: "Ma Ma, if I want you to be a grandma, is that mean I should be an adult ?"

I said: "right, but not just that you have to get marry with a lady so that your wife can deliver a baby then I can be a grandma."

My younger son said: "But Ma Ma you already xx (I don't how he can find out how old I am), I have to be adult and get marry then by that time you will be died already ?!"

I said: "No, you can get marry sooner so that you can have a baby sooner then I can be a grandma a little longer, right ?"

My younger son didn't reply but he started to cry instead !!!