Stereotypes are actually useful. Not all of them, obviously, but they have arisen because we actually need to make judgements and decisions in various circumstances. On the whole, provided they are not used improperly (as in being deliberately harmful) they serve useful purposes.
The emoji in question give a quick impression of an old woman (skin colour can be chosen from the one shown in the OP to all shades up to very dark skin, or you can go neutral as in the golden one). Nobody is going to mistake this image for anything other than an old woman.
Granted not all grandmothers are old but most are on the older side of life.
It's not a bad image. She has bright eyes, a smile, and a pleasant face. It's not a harmful image and a quick glance tells you what it's about. Quick comprehension is the point. Detailed pickiness about grey hair etc is just, well, pickiness to no good purpose.