lateral resistance band walks.

In actuality, this is one exercise pretty much anyone could stand to do, and makes a great squat warm-up. The glute medius primarily controls abduction/adduction of the femur, and its function tends to decline with age.

Also a more functional way of training the gluteus medius, which is the one you want to hit here. Look up good form, or you risk facilitating hip flexors and the piriformis, which you don't want here.