Bryce is right - this is an accounting mechanism. How old a loan is depends on total payments made not when they are made. For example, if a loan is 90 days late and then a regular monthly payment is made, the loan becomes 60 days late. Then if no more payments are made in 60 more days it will be charged off.
But as Bryce points out payments do still come in on charged off loans although it doesn't happen that often.