If the last payment was in May of 2014, then June of 2014 is when the account became deliquent and never paid again (until recently). They honestly should have had the information to give to you. If you grab your credit report from https://www.annualcreditreport.com/index.action , it will usually tell you the Date of First Deliquency on the account.
This would fall off your report in June of 2021. This won't change the date if falls off because you paid, and it's also possible they won't even show up on your other two reports if they aren't there already. This can be different for everyone's situation however. In the past year I cleaned up a total of 3 collection accounts, and 2 of them didn't report to all 3 agencies for whatever reason.
As far as the account's effect on your current Experian score......You won't notice an immediate recovery of all lost points. It's gonna take a bit for your score to recover unless you get this tradeline deleted from your Experian profile. It wouldn't hurt you to reach out to the Collection Agency again and speak to someone else for a goodwill deletion of this account. Also, if you suspect the reporting on the account is innacurate, you have the right to dispute it. You might get lucky and have the account information changed in your favor (deleted).
Thank you for the reply. I pulled 2 credit reports from annualcreditreport.com (Equifax and TransUnion, which are my higher scores). They showed no collections. When I attempted to pull my Experian report through acr.com, I continually received an error message, so I signed up for Experian's website and pulled all 3 there as well. That's where I discovered that my Experian score is 626. On the full report, it only shows one date in relation to the collections account: March 2018, which is listed as "date opened". There are no other dates on the Experian report, and the only date provided to me by phone when I called the collections agency was May 2014, which they described as the date of the final payment to T-Mobile.
From all of this, I think it might be safe to assume that - as you said - June 2021 is when it'll drop off my report. I'll just have to be patient until then, and I'll hope that the collections account is not reported to the other two agencies. My main disappointment is that I was thinking about applying for a mortgage on a small, affordable condo within the next two years. It looks like I'll need to wait at least two years now. Then again, I'm not familiar with mortgage rules and typical lending behavior. Maybe I could still get approved since I have a high income, two high credit scores, 0% credit utilization (of approximately $45,000 available credit), and zero debt (no credit card debt, no student loan debt, no other debt).
Any thoughts on how this might affect my ability to get a mortgage approximately one year from now? Thank you all again.