Just received my 8th anniversary badge on May 16. Most Simmers who created their account at the beginning of June will show the date they joined on. The reason my account shows a date before TS4 was released is because I created the account at the TS4 Creators' Camp, among other Simmers. So if you see anyone with a May 16 joined date you will know they were at the TS4 Creators' Camp.
Though I have an 8th Anniversary badge, I have actually been active with the forums for about 17 years.