I am 60. Back in 1965, I dislocated both knees (for the first time) at the same time playing football. I've had two surgeries on my right knee and one on my left over the years.
My knees progressively got worse until I discovered the Nordic Track ski machine six or seven years ago. Two or three 20 minute workouts on it a week actually strengthened my knees, particularly the really 'bad' right one.
A year ago in February, I bought a Hobie AI. To my delight, the Mirage pedal drive strengthened my knees too, much like the Nordic Track but it was a lot more fun with a lot more entertaining scenery to watch. Btw: I also use the largest Turbo fins.
Go for it.
"You'll only regret the things you didn't try."