As of today (Dec 28, 2015) I do not see any email in my gmail account older than 2006. How can I check older emails? I checked "All Mail" folder as well as other in my Gmail and none of them show messages older than 2006.
You probably do not have any mail older then 2006. Gmail will display all mail irregardless of its overall age. After checking my own account which was created in November of 2004 (only 7 months after Gmail first came out). I can see all mail going back to the original welcome email. See the screenshot below as evidence.