Like mentioned above it all depends on what YOU are looking for. Some 60-70 years old like to cruise with a mixed age crowd, while others like to try and cruise with mostly older adults like themselves.
More than just a specific cruise line, cruising options like cruise itinerary, cruise length and time of the year will greatly affect the overall demographics on a cruise.
You will typically find the younger average cruising ages on: Caribbean cruises, shorter cruises, cruises during school breaks (including summer)
You will typically find the older average cruising ages on: Alaskan cruises, European River cruises, American River cruises, World cruises, longer cruises
Take a 3 day Caribbean cruise over spring break: average age may be 28 years old.
Take a 8 day Caribbean cruise a couple weeks later on the same ship: average age may be 48 years old.
Those are just some generalizations that aren't always true, but for the most part they should at least give you an idea of what can be expected and what other options to look at.
Whatever cruise you take there will be others your age to interact with.