Cherry I know what she meant, and I know it's not easy to judge, but on the whole I think an able bodied adult is better equipped to stand up safely on a moving vehicle than a young child.
The bus we were on was one of those bendy buses and we were on quite windy roads, coming from a park and ride to York city centre. The bus kept jerking and it was difficult for everybody standing.
I didn't expect anybody to stand so my son could sit down, but it was hard for me and harder for him to keep his balance all the same just because of the type of bus, the roads we were on, and the driver seemed to have a heavy foot on the brake and accelerator.
I don't use the bus much now but if I saw a child struggling like that on a bus I would let them sit down, just because I think I could manage to stand more easily than they can.
I don't think the argument that when we were kids we were expected to stand up for any adult really holds up now. That wasn't because of hidden disabilities, it was more that children were somehow lesser than adults and the adults were judged to be more deserving of a seat. Men would stand for women, children would stand for adults. That was the way of things then but it doesn't mean it was because children were safer standing on the bus than adults were.
I'm glad you didn't fall, but I'm sure plenty of others did. And that was my point, children might be younger but they're still not that strong or steady on their feet, especially on a journey that isn't smooth.