I'm 58, and have had both knees replaced...the left was done five years ago, and the right was done almost two years ago with a revision done almost a year ago.
I went back to work both times after four weeks, as long as I was being cautious...I'm on my feet a lot, with occasional kneeling and crawling under desks.
I think I started riding cautiously after six weeks the first time, eight weeks the second. As long as you're not really hammering it hard and are using your gears appropriately, you'll do okay. It's the surrounding tissue that you have to be concerned with, not the artificial joint itself.
Also, I don't know how widespread it is yet, but my right knee was a press-fit into the bone, rather than being cemented in place. The pieces have barbs on the spike of the metal pieces to prevent them from coming loose. The cement is the main point of failure on artificial joints, so this is a good thing. On the other hand, the press-fit joint hurts a lot more in the short term...they literally hammer the things into place, and I'm told it's pretty revolting to watch.