Foster care and Adoption

Hi,i'm new here :) i just had some questions i wanted to ask about foster care and adoption. i dont wanna put my whole lifes story out lol but some is good. well to begin with,my mother was a foster child when she was young. her and my uncle both went from foster home to foster home until they were placed in a(i guess) permanent foster home with who i now know today as my aunt. my mom wanted to give back for what she had gone through in foster care so when i was about 7 or 8 she took in 3 foster care girls. there were a total of six of is,i know crazy. to this day those girls still consider us their family and they only stayed with us for about 2 years. since then i wanted to be a social worker,but after a while that changed and i wanted to become a foster parent, or a foster to adopt parent.i have become so passionate about it,and researching, however because of age i cannot start yet. Most teens(im 18) cant wait to be 21 so that they can go out and drink,but i can honestly say i cant wait to turn 21 so i can sign up to finally become a foster parent. with all that being said,im finally getting to my question. as i was doing my research i noticed that a lot of the agencies require you to be 21 to be able to become a foster parent,but 18 to be able to adopt a child. why is that? thank you for your input