I think the right skirt length depends on the overall outfit. No question you have the shape to wear practically anything. You carry off the bikini like very few 50-year-olds (I'm late 40s myself) can, and I'd keep wearing that if I were you -- because bikinis are classic. On the other hand, the black shorts outfits feel a little too short to me. Not because of your age, but because the overall style feels slightly dated, which in turn can date us when we wear it. So in terms of fashion, shorter is not always better, even if you are showing gorgeous legs.
In the last photo, I would prefer to see some cleavage or some leg as opposed to showing a lot of both at once. Again, you look fantastic, but the outfit does not feel modern. I'd love to see you in a skirt or dress above the knee, or perhaps something with a slit. Classic and form fitting, but still sexy. Like a Catherine Zeta Jones, for example. Or Sharon Stone. Or Kim Cattrall. All have tremendous sex appeal in a modern and classic way.
Thank you for the post. Very inspiring for all of us who want to continue looking our best into our 50s and beyond!