YouTube's UI prominently features search. So if my interests change and I search for something I've never been interested before, YouTube can show me a pretty good selection of videos and tune its recommendations for future viewing.
I will say, though, that all social networks seem to fail at weighting past behavior too strongly. Whenever I start a new account on a social network, I find it's recommendations are much more tuned to my interests, and those recommendations become less relevant overtime.
Biologically, I'm a new person every seven years. As a collection of varied interests, it seems like I turnover much more often than that - and I suspect the rate of turnover is different for everyone.