Depends on the moderator, as some might not see the benefit of removing older posts which are unlikely to be seen. Although, I can definitely see the logic in that, I usually just handle them anyway, if it's spam, it's spam, and probably needs to be removed.

Plus, a lot of threads have been active for years, especially those in the altcoin section, so those threads are likely viewed by hundreds or thousands of people each week, so it makes sense to clean it up.