One way to deal with the "stale data" problem is to prune old users from the system, but many people will be understandably dissatisfied with this, and it is not recommended.
I don't think you should worry too much about this. Someone who abandons a free service for an inordinate amount of time shouldn't expect to see their data. Especially when the cost of storing such data isn't nominal.
Send them one or two reminders and you should be fine.