I am 52 and am now in my third month of CF. I had a running background, but no strength training. They work with me to modify what i can't do (pullups for instance), and show me & have me demonstrate proper form for all weight lifting before doing the exercise with weights. I have gained strength, and my last 5K was 2:11 faster that the one done just prior to starting CF. So it can help folks of all ages...
Tim Ferriss writes about a 70yo guy doing power lifting... which as I understand would be similar workouts to crossfit.
woman in her 90's setting records... I think you'll be fine doing crossfit