Grandma?? I don't see grandma! Maybe if you are wearing these sandals with a cottons sweater with puppies and beach balls on it. Otherwise, I think you're fine
Being 31, I find labeling you "grandma" rather interesting. I speculate it's due to our over-glamourised, hollywood-ised culture that expects full face of makeup, jewelry and heels with every outfit. Which is strange because we are also an extremely casual society that celebrates t-shirts paired with little shorts and flats.
I think in the end, it's what you pair it with.