Timely thread. My Michelin LTX M&S LT265/75R16 tires (predecessor to the Defender line which has now officially replaced them were made in DOT 1813) -- 9 years ago and they were mounted within weeks of being made. The truck has always been parked outside in mostly a shaded spot (say 60% of the time).
These tires have travelled less than 80,000km (50,000 mi.) and have lots of tread remaining 10mm - 1/4" with even wear as I rotate them regularly and balance if needed. Up until last Autumn 2021, there were no visible signs of checking (sidewall, tread, anywhere etc.)
This Spring it's all come out ... the checking that is. There is a visible ring around the middle, outside of every tire, spider webbing cracks between this ring and the block edges and cracks developing in the outer tread channels.
If I was just driving around town or regularly light loaded like I've been the past 2 years, I might be inclined to leave them on until Fall. Since we are planning some major camping outings this year (finally), there are being replaced shortly. I believe a big part of the cracking becoming prominent is the fact I've only put a few thousand miles on this truck the past 2+ years.
I have no closely held rules regarding replacing worn or aged tires on any of my vehicles including our motorcycles other than I do so when it it time (wear, damage or visible deterioration). Regular inspections and common sense dictate my decision making. I will be posting up another thread on tires shortly --- this time for replacement recommendations.