Why does an American citizen have to be over 21 years old to sponser their parents?

Question

I understand that but why 21? Why not 18?

Answer 1

because foreign parents who made sure to give birth here in the US will use their infant as a means of getting an expedited immigration status.

EDIT: because most likely at 18, you're just fresh off high school. you do not have any means of supporting those who you want to immigrate. at 21, most likely you've been working and saving up money.

Answer 2

That is what the age has always been and will continue to be.  Are you able to prove, financially, that you can support your parents in every way so they do not TRY to get aid from the government?