Would slick tyres make cycling easier?

Smooth tread tires will reduce rolling friction on any bike. However, if you bought a mountain bike and ride exclusively on the road you probably made the wrong choice. When you ride on a smooth surface you have no need for the energy robbing suspension of a mountain bike. The lower gearing that is needed for off road conditions is of little use on the road. When you switch to narrower road tires, they will also have a smaller diameter which will also further lower your overall gear ratio. 40 is not old for a cyclist, I have many friends who took up the sport at that age and became strong riders, even graduating to competitive racing. If you continue to ride, your fitness will improve, but it will take time. As your fitness improves, riding will become easier and your enjoyment will increase. All it takes is that you continue to get out as often and as for long as you can manage