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Unfortunately, Maryland has a very short statute of limitations for breach of contract claims (it is 3 years for oral contracts, and 3 years for most written contracts). See: http://research.lawyers.com/maryland/maryland-statutes-of-limitations.html (Different states have different statutes, see: http://www.nolo.com/legal-encyclopedia/statute-of-limitations-state-laws-chart-29941.html, and that may be where the 6 year number came from).
If you have not been paid since 2009, that would mean that you cannot sue the individual for the debt, however, you can still try to get them to pay you by sending them written or oral requests for payment, etc. It just means that you have no way of judicially enforcing your loan.
I am sorry, I do wish I had information that was more helpful for your position, but I want to provide you with a clear and concise statement of the law so that you can plan accordingly.
If you can show that your promissory note was "under seal" - then a longer statute of limitations (12 years) would apply, see: http://mgaleg.maryland.gov/webmga/frmStatutesText.aspx?article=gcj§ion=5-102&ext=html&session=2015RS&tab=subject5.
Here is a helpful discussion of what this very critical distinction is: http://www.harrisonlawgroup.com/hlg-updates/what-does-e2809cseale2809d-next-to-your-name-mean/