Age

I was thinking about age today, and about riding the bus, and a question came to my mind that is not too relavent to me yet, but will be one day.
At what point does getting up and offering your seat to someone else on the bus become unnecessary? Or does it ever? Or is there a heirarchy? Like men get up for women; young women for older women; young women for (much) older men; everyone for pregnant women?
Just wondering.