Charged off accounts are often bought up for pennies on the dollar before the SOL is up and sued on. If the collection agency sues you and obtains a judgment, they can enforce the judgment by levying on your property or wages, subject to certain restrictions depending on what state you are in.
In NY, a judgment can be collected on for 10 years, and at the end of the ten yr period, it can be renewed for another 10. So the period of collection on a judgment usually ends up being 20 years. This is why you never want to ignore a lawsuit, even if you owe the debt and have no excuse for not paying it.