The few I checked were around 90 days old. I wouldn't be surprised if I looked long enough they'd circle back or link to a known spam account.
There's about 1600 accounts in my suspect list from this last week that need hand checked to weed out the "good" apples. This is just from the query I run to find those that are commenting with their own or bought voting trails. I didn't get to the other 2 queries I usually run.
I stopped before I spent my day on this.