I’m a tennis player who has played pickleball several times this summer. It’s a lot of fun and I plan to play more. Definitely see it as an activity for H and I when he retires. Went to see my dad in the Villages and there were tennis courts, pickleball courts, platform tennis courts and a beach tennis net. There were people playing each of the games at 10am, 95 degrees, 90% humidity. I loved seeing all the activity!
Looking at the pics on our local pickleball association there are people of all ages - looks like lots of young men in their 20’s, 30’s, 40’s! Women too. Also older people.
I asked my S who played tennis in high school and college if he knew of it. He said “omg, our elementary school gylm teacher used to always make us play - I hated it!!!” So sometimes for the VERY young! 
"My plan is to take it up when I can’t play tennis anymore. "
Ditto.
Several of my tennis friends also play pickleball now, too.
My friend’s husband recently retired and has taken it up. He’s addicted now.