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It will very likely show up on your report (and cosigners) as a collection account. Collection agencies use your credit as leverage to get you to pay them since a bad account hurts your ability to apply for credit. DOFD would have only counted if you havn't paid the loan since those lates if you were trying to see if the loan was beyond the SOL's for your state.
It a bit surprising that the lender stopped reporting on the loan as it was still current at the time.
Have you tried to see if the lender can "get the loan back" from the collection agency? Try seeing if you can arrange something with the lender and not the collection agency.
The DOFD is only relevant for defaulted accounts - and it is the date when the account first fell behind and was never brought current again before being placed in default status. So, if you were late 8 years ago, brough the loan current, and then went late again last year which lead to the default, your DOFD would be last year. Usually, when an account goes into default, it's charged off or transferred to a collection agency, so that tradeline closes and falls off your report entirely 7.5 years from the DOFD.
If you just go late and the loan never defaults, those individual lates remain for 7.5 years, and the loan itself, if it closes in good status, will remain about 10 years (so you can wind up with a window where those lates are gone, but the original tradeline is still there and now completely positive).
I'm not sure why the original lender stopped reporting this loan if it was actively being paid or in deferrment the whole time. From what you've said, it sounds like the collection agency will be able to report it on your credit - this loan is likely still within the statute of limitations because you've made payments and/or kept the loan current through deferment until recently. Also, it sounds like your DOFD would be whenever the loan went late after your in-school deferment should have started but did not. They can (but may not) also report all of this on your co-signer's report.
I would also suggest contacting the original lender. In light of the fact that you were in school and the loan normally updated with that status on its own before, you may be able to argue for a retroactive deferment to bring the loan current and remove the lates.