Less hard muscle to absorb the impact, more brittle bones. Your technique can be as good as ever or better but we'll still risk injury on any fall at our age. Part of it's also a matter of preferential memories I think. I stepped off a running board to the curb and injured my foot in my 20's, and my falls were more bouncy and ungainly until I got better trained - in my 40's. We don't need to just give up on it on account of age, but we do need to be more careful.