'Tend to' implies that the condition you are describing is likely for that group of people. Tendency is the associated noun form. So if you say 'Young people have a preference ....' you are implying that all young people do. If you say 'Young people rarely have a preference ... ' you are implying that only a small percentage of young people do. If you say 'Young people tend to have a preference... ' you are implying that (significantly) over 50% of young people do. It can be used with many characteristics and with individuals as well as groups. I tend to fall asleep if I watch a boring movie. People tend to get less fit as they grow older unless they exercise.

It means that they are likely to have a preference of exciting activities, or would rather choose them over something else. tend (BE LIKELY) /tend/ verb [I] to be likely to behave in a particular way or have a particular characteristic: [+ to infinitive] We tend to get cold winters and warm, dry summers in this part of the country.