I asked someone in Onepass management about this and received the following response:
The best thing to do is have some kind of activity in your account every 12 - 18 months. This protects you from having your miles expired and also keeps you in our active member database. Being active in the program helps you stay in touch with what is happening in OnePass and improves your chances of getting a targeted promotion.
To date, we have not expired miles due to lack of activity, but things can change. Therefore, we have the language in place in our T&Cs.
The date the account was created is not a factor.
(emphasis added by me)