What is considered really good length of credit history?

[webhopper]

Most FICO High Achievers have an average age of accounts between 6 and 12 years

[guiness56]

I'm not sure about the DMP.  But if those cards were closed in good standing they will report for up to 10 years from date closed.  If they are adverse due to late payments, each late will come off 7 years from their occurence and when the last one is removed, it will again be a positive account and stay for up to 10 years from date closed.