Cripes! I've lived in a bungalow since 1985! I visit my daughter about once every six weeks and the stairs in her terraced house aren't a problem! I've never even thought about it.
I'm 72 and I take the stairs at the leisure centre, at a run, two at a time, twice a week, just because I can! There are plenty of ways to keep your legs strong. Squats and lunges are great. We do both in our karate training. Brisk walking will help. I also carry heavy shopping bags with the handles at shoulder level rather than down at knee level. If I only have one heavy bag I count to twenty then swap hands. Try it, why not? Start small and increase slowly. Walk a little faster for a count of twenty and alternate with a slower pace.
Unfortunately, the secret to staying fit and strong in your older years is to start, and keep going, while you're younger! I took up karate when I was 44 years old and have never stopped. Complacent? Moi? 😎
However, you're younger today than you will be tomorrow so why not start, today! Never give up!