I'm female, 54, healthy looking afaik . Not overweight, so unlikely to be mistaken for pregnant, even if I was young enough, I was wearing comfortable shoes, no heavy bags.
I'd been stood for a couple of stops when the woman in the seat nearby jumped up and insisted I took her seat. At the time I assumed she was about to get off but no, she stayed on several more stops and took a seat when one became available.
She must have been at least as old as me
So, what was that about?