Walking barefoot on a nice lawn or a sandy beach - nothing nicer. Indoors, on hard floors, wood, tiles, concrete is a risk I won't take. Plantar Fasciitis is very painful and limiting and often starts for someone with dropped arches, flat feet or overpronation, walking barefoot on very hard and unforgiving surfaces. Problems with pain in knees, hips and feet can then follow.
Young feet are likely to be much better padded! Muscles and joints stronger. The general advice is NEVER to wear any shoe or sandal which is completely flat. It's the flatness that causes the problems, especially forcing a bare foot against hard surfaces.
My everyday ( indoor and out) sandals have always been Birkenstocks, and clogs if I am working outside. Lots of very lightweight sandals to wear for a change but always with a padded/shaped insole and a raised sole. I had P.Fasciitis once - years ago - and that once was enough !!