6th or 7th?

 On Facebook someone was talking about completing their sixth decade. They were 69.  I didn't say anything, but you are ten when you complete your first decade, and 70 when you complete your seventh.  I am in my 7th decade now, at 64.  I really don't know how this person was 11 years off.  Perhaps it is because your 60s seem like number 6, so your 70s (your 8th decade) would begin at age 70.