HELP! Old acct with AMEX since 2001. Think Statue of Limitiations is up.?

Just last week i received a letter from AMEX regarding an old account i believe was charged off years ago. It said that the new balance was some ridiculous amount that was 2x's the balance. They offered me a settlement for the correct amount for 50% which i agreed to. I just want them off my back for good. They told me to fax them an authorization settlement letter. It includes my name, fax number and says "i authorize AMEX to send settlement letter for payment of balance due on sept 30th. Did i just reset the SOL by doing this? Is this just as good as saying i accept the debt? I think it does but need an honest opinion. Thanks for any advice.... Al