My wife & I are just back last week from 2.5 weeks in France, (the first week spent in Paris). It's our 4th trip to France & we always begin with a week in Paris. We've used Guest Apartment Services every time, for a rental on the Ile St. Louis. As has been said many times on this & other forums, L' Ile is, in essence, a small village plunked down right in the middle of the Seine, Groceries, bakeries, bistros, restaurants, are all, at most, a few blocks away. (including the worlds best ice cream parlor). Both right & left banks or Ile Cite are a short stroll over one of the several bridges, (each with great views). Metro stops for destinations on either bank are adjacent to the bridges, for sights too far to walk to. We've found it the absolute perfect place to venture out from each day to explore & return to each night to rest.
Guest Apartment Services, has been outstanding for providing great places en 'L Ile... airport transfers, & good concierge advice/service. They are not the cheapest option by any measure, but we've found that this is where stretching the budget gives us the most satisfaction. We're both in our 70s as well, BTW.