In the olden days when I was child, we never took a seat on a bus or train while there were adults standing. I was dismayed today to see children pushing past adults to get empty seats. We were on a packed train, adults sqashed in like proverbial sardines, and there were so many apparently able bodied children sitting. I can understand why toddlers might be safer sat down, but these were from pre-teen upwards.
On the plus side, my elderly mother-in-law was always offered a seat, not from the youths however but from nice gentlemen.