Typically, young male runners obsess over their mileage counts and their weight. Most avoid lifting like the plague. This makes sense since races are won by ectomorph body types. Young runners also tend to not eat much protein because it is expensive and they don't have much money to spend on food. When you are at your physical peak, you can get away with a focus on just running. My teammates and I all did this in our 20's.
Aging guys have a different mindset. As you get to your 30's and 40's, you notice that your physical strength and flexibility diminish. You also see changes in your balance and motor skills. Worst of all, you start to put on weight and your ability to recover diminishes as your hormonal balances change. Lifting, yoga, and cross training are good ways to minimize those declines and older guys gravitate to those activities since most can't or won't run huge volumes any longer. You also see what happens to your unfit colleagues and aging relatives that don't embrace fitness - chronic diseases and falls/fractures that often hasten their inevitable demise.
Older men also have secure incomes and can afford to eat better. While the struggling post collegian gets by on cheap carbs, middle aged athletes can afford more fresh produce and meat. The increased protein intake helps with muscle development and being lean accentuates how that muscle looks. Now, a 50 year old man won't get back to his stick figure college track/XC fighting weight ( and we know that we won't approach those times either) but, if he is carrying only 10-15 additional pounds and half of that is muscle, then he will look like a beast compared to a 125 pound collegiate runner.
I grant that there are some masters athletes that are using illegal substances or that go to a "Men's Clinic" to get testosterone therapy that they really don't need. They are outliers compared to most of the better masters athletes that have stayed running fit while doing the strength work and eating a diet that insures better health later in life.