Exercise doesn't all have to be done in the gym or require extra equipment. Your body alone can provide an effective workout stimulus throughout life as you progress through more advanced moves and play with body leverage.
At age 56, I get to be a rock star. In an athletic setting. Surrounded by some of the fittest and most motivated people I have ever met. All I have to do is keep showing up and let them watch me go at CrossFit with complete abandon. I have a new set of friends and admirers.
So you may be asking yourself, why even bother with pull-ups? Putting it simply, the pull-up is one of the most effective upper body exercises you can perform. It's not called the upper body squat without reason.