Her own idea was that, if she lived long enough, she would become a spinster.

Unless someone took pity on her when she was old and decrepit and past her work.
There was a little humorous smile about his mouth.

But his eyes were serious and pleading.
"When shall I know that you are old and decrepit ?" he asked.
She was not quite sure.

She thought it would be when her hair was grey--or rather white.

She had been informed by experts that her peculiar shade of hair went white, not grey.
"I shall ask you to marry me when your hair is white," he said.