Its much more of a hassle concern than it is a realistic inability to climb multiple flights of stairs. Manhattan would be a really, really terrible/uncomfortable place to live if you couldn't physically navigate at least a few flights of stairs at once between subway stations, split-level stores and restaurants, residential buildings, etc, etc.
Just thinking anecdotally, myself and many of my neighbors in NYC are probably dealing with a dozen or more flights or stairs up and down on a daily basis (two flights down out of the apartment building, half a flight up to the street level from the sunken entrance, another 2 flights back down into the subway station, 2 flights up on the other end of the train ride, a flight in the grocery store, two flights at the gym, etc, etc, etc). Escalators are often present, but many people walk up those anyways. I'd feel comfortable saying people living here are generally in far, far better stair-climbing-shape than most of the country.
Just thinking anecdotally, myself and many of my neighbors in NYC are probably dealing with a dozen or more flights or stairs up and down on a daily basis (two flights down out of the apartment building, half a flight up to the street level from the sunken entrance, another 2 flights back down into the subway station, 2 flights up on the other end of the train ride, a flight in the grocery store, two flights at the gym, etc, etc, etc). Escalators are often present, but many people walk up those anyways. I'd feel comfortable saying people living here are generally in far, far better stair-climbing-shape than most of the country.