Whats the oldest you can be to be a surrogate mother through an agency and how much do you get paid?

do these mothers without using your own eggs make a profit or is against the law?Whats the oldest you can be to be a surrogate mother through an agency and how much do you get paid?
nothing is against the law when not only a child is involved but money im sure it happends every day people are greedy im not sure about the laws but i had a friend whos parents paid another lady like 50k