There is nothing better than to teach a beginner, at any age, how to play tennis and then one day they show up and they are stroking the ball, moving and having "just fun" playing.
It is "just fun" for me to watch my students controlling the ball back and forth, taking balls in the air and exhibiting excellent control. I see big smiles and a lot of laughs and giggles.
From feeding balls exactly to the student and then watching them feed balls to each other and moving all around chasing the balls is a joy.
Then the first time they play in a social event, a tournament or team tennis, they are hooked.
Still, the biggest asset of playing tennis is how long you can keep playing. Plus, it's "just fun" to play, have a social outlet and keep fit!
Good luck on the courts!