size verse's age

Went to the lake this afternoon [1st time in 3 weeks ]and caught a lot of
small bass . they ranged in size from as small as four inches to keeper
size [15 in. ] and every where in between .Is there a rule of thumb about
the size and age of the bass . I figure that a 4 inch bass has to be
hatched last year and a 8 - 10 incher 2 years ago 12 inches 3 years ago
and a 15 inch fish maybe 4 years ago.does this sound about average???I'm
sure that down south the growing season is longer so the fish will be a
little bit bigger per year I am located in northeastern Kentucky
thanks
Ken Blevins