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Despite what they say in the advertising (30-50 age group) the age range of people actually on the ships ranges from young children to people in their 90's. If you go out of school holiday periods you will find a large number of passengers of your age. Organised excursions cater for all ages and abilities and I have known plenty of people in their 60's go on the bike trips.
As long as you're prepared for the companys young outlook and accept that everything isn't designed for older tastes theres no reason why you shouldn't enjoy yourself on the cruise.