With ageing and after weight loss the buttocks may loose shape and their nature curviness because of a combination of loss of volume and loose skin. This loss of shape can make the buttocks appear saggy and flat with a loss of projection. Exercises that can build gluteal muscles such a squats can help regain some definition by adding gluteal muscle bulk volume.
If there is no loose skin in the outside thigh or buttock area, then regaining volume in buttock region can be achieved. non-surgically with Radiesse or by fat transfer in a Brazilian butt lift with fat taken from other areas of the body using the body jet liposuction system. Importantly, liposuction in the waist area (Love handles or flanks) will have an impactful result in shaping your waist to hip ratio.
If there is also loose skin in skin in the outside thigh or buttock area, then a body lift is indicated in order to lift the buttock skin and also outside thigh. This can be combined with fat transfer at the same time.