What is a "generational shift" and why doesn't it mean that younger people should give up their seat to older ones? I stopped being an under-18 in 1958 and nowadays I'm grateful if someone does offer me their seat, though if my journey is a short one I may thank them and decline the offer, especially when the person making the offer is a good deal older than 18.
Offering a seat to someone considerably older is polite and civilised.
I agree with the OPs concern that, apart from clarifying a particular sanction (withdrawal of concessionary travel) that only applies to under 18s, the list of expected behaviours published by TfL should apply to everyone, and the way they've sought to make their point is clumsy and not properly thought out.