Wanted to post this in WWYD but I couldn't find it for some reason...
I'm late twenties and able bodied and I normally avoid the seats on the bus reserved for those less able to stand.
Today I was very tired so plonked myself down as no one appeared to have a need.
A lady then got on and stood quite near me. She didn't have any walking aides or anything and I'd guess she was around 120kg.
Would you have offered your seat? I was worried I'd hugely offend her if offered because she looked only a little older than me but I guess it must be more difficult for her to stand than me?
Thankfully only had two stops to worry and then scuttled off. 
Can't work out if I'm a git for not offering or if I'm a git for considering it!