One of the reasons why older tennis players transition into pickleball is because of movement and court size. Of course, because the tennis court is much larger, it requires more movement. The tennis court easily requires four times as much area to cover as a pickleball court.
 
So, what to do and when to do it? As you age you will feel like everything about you is going slower, especially your feet and movement speed. As a tennis player, if you love tennis as much as I do, then you must work more on anticipation. Instead of thinking you should move faster, try thinking that you need to start sooner. Start thinking in advance, "Am I in the offense or the defense."
 
What I think about in tennis is to be ready to move your feet as least 4 steps in every point before you give up, versus pickleball where I would be ready to take two steps to every ball before giving up.
 
You will definitely get a more aerobic workout with tennis. They are both fun and both social. I would recommend staying in shape, stay with tennis until you lose your foot speed then gradually go back and forth between tennis and pickle ball. As you lose your movement then transition to pickleball.
 
Good luck on the courts!