Scanned the T&FN Archives a bit, and found the first Bolt cover to be the Sept issue of 2003.
This gets tricky!!
He was born August 21st of 1986.
That issue probably went to press in late July or very early August.
Meaning Bolt was THEN 16 years and 11 months and change!!
Looked for Mary Decker when she was 14 or 15, but she didn't make the cover then.
(Though I think there was an article on her inside at that time!!)
Thus, Mary Cain, at 16 years and 9 months (and about one week) MIGHT be the youngest athlete EVER to make T&FN's cover!!
GH, do you know if there was anyone younger??