I'm a noob playing my first world and I have healed my first two zombie villagers today. Threw plenty of food in their "house" and got 10 baby villagers so far. Yet even after standing next to them for 20min+, their appearance does not change to adult. Total playing time since babies is couple of hours now. I suspect it might be a graphical issue as I had two iron golems spawn so far.
I have recently upgraded from 0.15.10 (I think), play single player only, no mods or addons whatsoever.
Also see issue MCPE-17517, you aren't the only one. Mine have been the same over days of playing.