Yup, what Ron said and/or they are over I believe it's 5 years old. Might be 7. Mine never get that old so I can't remember for sure. There is also the redneck depth gauge of using a penny. Lincolns head toward tire within the tread and if you can see the top of his hair it's time. Not accurate but better than eyeballing.
Are you looking at both tires? "Usually" the front tire can last twice as long as the rear, at least that's what I've experienced and most of my riding buddies have confirmed.
Don't feel obligated to change the front tire just because the rear is worn down, unless you've got $'s to burn. As long as the front tire passes the tread depth test described above, then just change out the rear one.
Is it really worth an extra 500 miles on possibly bad tires? Or put another way, if you spend $450 on new tires mounted etc., is that cheaper than the medical bills of an accident or the down time to heal?