Display template section only on older posts’ pages?

I just moved back to WordPress from Drupal, and am having trouble figuring out how to do this bit. I want to put Adsense ads on my individual post pages, but not on posts published in the last month. The Adsense plugin give me the ability to not put ads on the front page, but I can only toggle single-post-page ads on or off, not with any kind of granularity.

Any ideas on how to have ads show up only on older content? I don’t want to inflict any of my actual readers with ads, but the 90% of people sent to my blog come from Google, so I’m OK with Google paying the bills to cover those visits…