My theory on Mary Poppins:
My theory is:
She was a very strict british nanny who dies while taking care of a child.
But she didn't go to "the other side", she remained as a spirit fulfilling her task of taking care of children. That's why she magical and timeless.
Bert was one of the kids she took care of as a spirit on Earth. Bert was a homeless boy who can never forget Mary and how well she took care of him.