I dont know if that person had a condition that prevented him from standing but the fact that he had on swim trunks and a towel draped over his shoulder and got off at the water park makes me assume he was able bodied.
And I believe the guidelines for being considered senior is 62...although I am not possitive about that. Thats not to say that a senior isnt capable of offering his or her seat or that they are not entiteled to be given the opprotunity to sit down over a younger person. It is a matter of decency and compassion for others.
You need to stop assuming things. I am perfectly capable of going to water parks and walking long distances and doing just about anything, but one thing I cannot physically do is stand still for more than 1-2 minutes at a time. I have a condition that causes my heart rate to skyrocket and I nearly faint in these situations. From the outside, I appear to be a perfectly normal, healthy 25 year old. Are you telling me that I too should have to give up my seat for you because you have a baby?