Older adults without mobility issues should be fine traveling all over Europe. 

Of concern is those adults that have difficulty navigating stairs. May hotels do not have elevators. Many train stations also do not have elevators — although some may have escalators, it is common for them to only have one which probably moves up or down depending on the degree of foot traffic. Finally, as you probably have experienced, non-functioning escalators are as common in Europe as they are in North America.

Another thing to be cautious about is cobblestones. They are very common throughout Europe. For those who are a bit unsure of their footing, cobblestones can be a fall hazard. 

Be on the lookout for bicycles as you cross the street. Also, be aware of where the bike lanes are — itis not always obvious.

Some tours can be quite strenuous. Check the physical expectations and requirements. Some tours may involve a bit of hiking, a lot of walking or extensive standing.

If you have only Medicare A & B you are NOT covered for travel outside the US. You will need to obtain additional travel health insurance.