Our 18-month-old has always worn Robeez (basically slippers, with very soft, single-layer leather soles). We were at an event recently with a bunch of other kids his age, and everyone else seemed to be wearing "real shoes": with flexible soles, but still, definitely soles.
Is there a cut-off for moving your child to shoes with real soles? Google is giving me conflicting information, and shoe manufacturers (of course) all have their own agendas they would like me to believe are the one true way.
We live in California, with no real weather to speak of, so his shoes wouldn't need to protect him against cold or wet. Just form a protective layer between his tootsies and the sidewalk.