So exactly what law are you referring to that specifies when debt is "too old to legally collect"? Because I know of no such law. I do know that there is a law called the statute of limitations which prevents them from suing you, but does nothing about telling them that they have to stop collecting, with the exception of Wisconsin and Mississippi where collection activity must also cease.
...if you read what BigA posted, you'd see that he stated that the statute of limitations can only be used to stop collectors from suing for a debt. That does NOT stop them from contacting you and asking you to pay - unless you live in Wisconsin or Mississippi!
Sorry, but I am extremely right. I even posted the names of the only two enlightened states in the US that actually do time bar debt when the SOL runs out. Now a debt collector cannot sue you for debt that is past the SOL, but they do not have to stop collecting it. Of course you can post your proof since you claim to be knowledgeable in the law and I will review it, and then post what the law actually says.
We should be careful not to allow confusion of "legally collect" with "use of the legal system to collect", nor to use the former phrase as shorthand for the latter.