Again, I've never found a time limit on bad debts. However, in the information I previously linked you to from the IRS Pub 550, it states:
"A debt must be genuine for you to deduct a loss. A debt is genuine if it arises from a debtor-creditor relationship based on a valid and enforceable obligation to repay a fixed or determinable sum of money."
This does not in any way imply that you can write off any debt without documentation, regardless of how old it is. IRS can, and often does, ask for proof for any deduction on your tax return.
So the loans that have paperwork stating a repayment expectation on the surface seem to fit the criteria better than a check that just has "loan" written on the memo line. Without paperwork to back anything up, IRS has no way of knowing if that memo line was filled in when you wrote the check or not, or if there were any terms set up to repay the loan.
My professional opinion here is that you may certainly amend back to 3 years (2007, 2008, & 2009 ) and amend any return that the debt/loan became uncollectible in. However, you must be ready to supply the proof that this was a loan, and that you had on going attempts to collect the debt/loan in the manner set up in the original loan documents, and why you feel that this loan is no longer collectible should the IRS ask you for it.
Remember, the information you provide as a statement supporting your personal bad debt loss (listed below)
A description of the debt, including the amount, and the date it became due,
The name of the debtor, and any business or family relationship between you and the debtor,
The efforts you made to collect the debt, and
Why you decided the debt was worthless. For example, you could show that the borrower has declared bankruptcy, or that legal action to collect would probably not result in payment of any part of the debt.
are minimum requirements. The IRS does not ask for a copy of the repayment schedule signed by both you and the debtor to be filed with the return, but they most certainly can ask for that, and if they do, and you don't have that, then they may disallow the loss.
To some extent, this may come down to how strong you feel your documents of proof are. If you think you have enough to prove that these were loans and not gifts, and that you tried all you could to collect them, then file for the personal bad debt deduction.
I hope this helps give you a clearer picture.