Get the age of a Linux install

I've often wanted to know how long it has been since a GNU/Linux install took place. I usually just checked out the root directory and made a guess at it. But I found this little nugget on rachelbythebay.com:

ls -ld /lost+found/

Her install (or whatever is left of it) was at an elderly 12 years of age. Mine's a ripe 24 days old... Maybe someday I will settle down.