No Name,
What are you worried about? If they haven't started a lawsuit and just bothering you with calls then its just an irritant.
You have the following going for you right now:
-retired <--translation a poor target for a lawsuit
-don't need a mortgage or loan <-- translation, so what if it shows up on your credit report.
Everyone has an opinion, but IMHO, whats the big deal? Seriously. If the calls bother you that much, just change your phone number to a non-published one. End game.
You are in your retirement years, if it was me I would be enjoying this awesome summer. You stated you don't have the money to pay so why lose sleep?
All that engaging them with letters and chatter about the SOL and trying to figure out when you can rely on it is a waste of time, and just going to put you on their radar screen. Trying to prove you a smarter then a bill collector is a waste of time in your case.
Cross that bridge of answering to a lawsuit if (a big if) it happens. Based on what you told me your probably have a better chance of getting hit by lightening on a sunny day.
Heres a couple of articles you may find enlightening:
http://www.totaldebtfreedom.ca/articles/article_8.htm
http://www.totaldebtfreedom.ca/articles/article_13.htm
Enjoy the summer.