I understand where Phil is coming from. Have also had abuse for trying to be helpful. Does tend to make you reluctant to offer in future.
Coming at it from the opposite side of the fence, I'll admit to being somewhat shocked when someone offered me their seat. I know I'm getting on a bit and wobbly ... but I was still rather shocked, and hope I wasn't offhand or rude in my "thank you but no thank you". But it *is* good that people ask ...
I can remember when someone first offered me their seat, several years ago now. I was probably in my late 50s and didn't consider myself to look "that" old at all, so it was a bit of a shock.
These days it happens more often than not when standing on buses, which is the norm on my local service. But strangely, almost never on trains, even though I frequently do have to stand in any train journey I make.