Does there have to be an age limit between when you turning 18 and when you go into foster care? Also can they deny putting you into foster care?

Foster care is not a right, in some places there are "childrens homes" where children may be keep if there is no foster care, also there are temp foster homes where you may move around from home to home.

At least in GA, at 18, the money to foster parents end and on your 18th birthday they open the door and tell you bye,

But often the system will do any and everything it can not to put you in foster care first, they work with parents and guardians to try and fix current issues