I ride the SEPTA bus to and from school every day, and I often see older people standing and younger people sitting. What's the etiquette for offering someone a seat?
I, too, ride the bus pretty frequently, and I like to think about how public transportation is one of the great equalizers (that is, for people who actually use it). If there's traffic, we're all stuck in traffic together. If someone's eating a smelly sandwich on the bus, we're all smelling it together. If it's crowded, we're all crowded together.
There are specific seats on SEPTA buses that say they are "priority seating," with a picture of a wheelchair. These are the seats with the wheelchair hookups and also the seats that are intended for people with disabilities. They're close to the front of the bus, they're labeled and it's pretty clear what you're supposed to do if you're an able-bodied young person occupying one of those seats. Here's what SEPTA's website has to say: "Priority seating is designated on all SEPTA vehicles by a decal indicating priority seats for persons with disabilities. On buses, two priority seats flip up to accommodate two wheelchairs, scooters, or wheelchair strollers. The operator will ask a rider to change seats if they are occupying a priority seat, but under ADA, other riders cannot be required to vacate these seats."
Your question, though, if I'm reading it right, deals with the gray area of someone who looks older than you but doesn't have another identifiable characteristic that would indicate a particular need for a seat. If you feel comfortable offering up your seat and there's any doubt at all as to whether someone would benefit from being able to sit down, go ahead and offer. However, if, as happened to me recently, the person responds with something like, "I'm not that old!" be sure to follow up by asking someone else nearby if he or she would like your seat, preferably someone young and able-bodied like yourself. That way, you just seem like a nice person and not like you're singling people out for being elderly.
SEPTA has a very nice list of "Passenger Etiquette" tips on their site, and I want to highlight two of the points:
Yield Priority Seats to riders with disabilities and seniors
Be nice and offer seats to parents with small children
One is dictated by the decals on the seats and an official protocol, and the other is part of riding on this great equalizer of public transport. Offer your seat because it's nice to be nice to people. There are few occasions we encounter on a daily basis where we have a socially-acceptable opportunity to be nice to a stranger just for the sake of it, but this is one of them.
A counterpoint to all of this is not to judge a seemingly healthy young person for sitting down when an older person is standing nearby because none of us ever knows what someone else is going through. A couple years ago, I hurt my foot pretty badly, and I couldn't stand on the bus. A couple of times, I had to ask someone to give up a seat for me, and it was a humbling experience bordering on humiliating, mostly because I knew people were judging me for sitting down when older folks near me were standing. It's also worth noting, though, that when I was pregnant, people would stand up and give me their seats without even asking first, and I always appreciated the gesture.
So enjoy the amazing civic experience of riding the bus, and if the sandwiches ever get too smelly, you can refer to two other points on the etiquette list:
Cooked or prepared foods don't mix with transit travel
Take your meal home to eat
Safe travels, and be well,