Tire replacement

Another maintenance issue question.

Noticed yesterday a small abrasion on the side wall of my tires.  I can see the kevlar webbing.  I figure it's time to replace.... yes/no?

I also noticed some cracking along the side wall the must have occured while riding on a flat (plus the arid climate doesn't help).  This cracking is on both tires (I road briefly on them flat - before I KNEW they were flat).  So I decided to replace both.

I would say they are about a year old with average miliage.

Thoughts?  Are they at the end of a life span?  Should they be replace due to the cracking?  They seem to be ok except when I keep hitting rocks or huge cattle guard bumps - (maybe I should slow down a little when I hit them - as a side note)