Aside from the above excellent answers, I sometimes throw in circuit style classes. Each participant will work at their own pace, but they need you to stress that they should work out at their own appropriate level. Take this example: I may have everyone cross the floor and do 10 wall push-ups, and then cross to the other side and do 9 wall push-ups, (and on down to 1 push-up). This is a mix of strength and cardio. While my ‘spring chickens’ will sprint from side to side and do their push-ups on the floor, my ‘seasoned vets’ will stroll more slowly, and use the wall for push-ups. Those done early are doing squats until the rest catch up. Everybody works at their own level.
Circuits with stations set up around the room are another way to let everyone work at their own pace, rather than a fast paced, choreography laden class that will leave a few behind, never to darken your door again.