When do tire cracks necessitate tire replacement?
 

My 2003 Civic has Michelin tires on it that were manufactured in April of 2012. They were installed in July of 2012. They have 25K miles of wear on them, and they are rated for 90K miles. The tires have some cracks in them (shown in the images). A mechanic told me that the cracks were severe, and that the tires need to be replaced. Should I believe him?

From the looks of your pictures, I hope you replaced your tires. Generally, it's not the amount of tread left on a tire that's the replacement factor, but the age of the tire. Everything I've read points to replacing tires at the 6 year mark to be safe. I have a set of BFG Radial T/A's on my 72 Corvette that have about 1,500 miles on them and they look brand new and have no cracking like yours at all. Only problem is they were purchased in 1996! I haven't replaced them just because the car sits in the garage and I start it once a week and drive it around the block and then back into the garage. For safety sake, if you use your car a lot, replace your tires.
Duane