There is no specific age limit for a BBL. If you are healthy and have fat reserves then you can probably get a brazilian butt lift.
While the average patient having a BBL is young to middle age, there are some older patients also requesting the procedure. In my office the oldest patient having undergone the procedure is 69 years old. You can have the BBL, even if you are not young, as long as you are physically healthy and are a candidate for the procedure.