I couldn't find my password on the list and I've been using the same password for LinkedIn since I registered. I was trying to remember when was that. If someone know how to find out the last time you changed your pass or when you registered for linkedIn please let me know. I'd guess I use linkedIn for over 4 years at least.
A "member since" date is available on the "Account & Settings" page. Choose "settings" in the drop down that appears when you hover over your (account) name in the upper right corner of any LinkedIn page.