There are some elderly people who walk with one arm across their back. And maybe this is what you saw and the lady was just chatting to the young girl.
Or the young girl was threatening the old lady.
Maybe the old lady had fallen and hurt her back? Then she put her arm up onto her back where it was hurting and then linked the young girl for support.
If you stopped every time you saw a stooped or elderly person on the pavement (accompanied or not) near me, you'd not get very far - there are OAP sheltered bungalows across the road.