Most popular cities for travelers age 65+

I'm not aware that any cities are more or less popular for the over 65's vs the under 65's. If there are mobility impairment issues, that's another matter (for instance, Venice could be quite difficult), but if you can walk at least a few blocks, climb a flight or two of stairs, etc, there's no reason you can't go anywhere a 20 year old would go. And if you can't do those things, you can adapt (taking buses instead of the Metro, for instance). So, what place draws YOU? What's on your bucket list? What cities have you already been to, and what did you think of them? Is there a country you've always wanted to visit - or avoid visiting? Do you have any particular interest that could influence your choice? For instance, Roman ruins are not just found in Rome, or in Italy for that matter - there are significant ones in Cologne, Germany, near Seville, Spain, and in Nimes and Arles, France. The answers will not only help us, but also help you focus where your interests lie.

John,, sorry , but two active seniors can go where ever they wish, and enjoy it too, if they choose based on their interests , not just others ideas. I would look at allotted time, ( one week, three weeks, how long??) and start with that. I am not a senior yet, but find that when I was younger (in twenties) I was happy going from place to place one or two nights, but now in the middle years ( ok , I am 50) I prefer longer spells in each place, less rushing about,, more time to see the sites in each place and still have time to sit in a cafe for an afternoon if pooped out.
I would also consider an open jaw flight( often sold as"multi destination" flights on websites). Fly into London( its the western most hub city for most airlines, so in interest of minimizing jet lag and travel fatique is a good first stop) explore as you wish in UK, then eurostar to Paris( take an apartment there for a week.. nice to make your own cofffee in the am) . then perhaps train or flight to Rome,, .. then you could fly home from Rome. Or instead go east to Germany or Switzerland, really there is no bad choices, choose what intrigues you! Just take it easy on timing,,, remember, when you schedule a two night stop in one place that only really leaves one full day at that destination, most places are worth a bit more then that ( unless you are truly in transit only). Good luck and do tons of web research, you can really get some great ideas from forums, this and others, but as Harold suggested, you have to focus on what you really want to do and see and then ask some more specific questions.