I started have trouble getting in and out of the car starting when I hit about 40 (yeah, I'm an old fart!)...and I figured out what caused it!!!!
That is when I got fat...not just a bit, but like added almost 50 lbs to my optimum weight. That is also when I started having trouble with my knees about two years after that...now it is the diet and exercise...well ok...exercise and careful about eating...diet sounds so 'Hollywood'!