All tires sold in the US must have a DOT (Department of Transportation) number stamped on the sidewall. Most people pay no attention to this and are just happy to have tires with descent safe tread on their cars, but there is very important information contained in this number. The plant that made them, size, manufacture and brand are coded in the first grouping of numbers. Most important to the consumer is the last four digits of the number that identify the date of manufacture. Take note the the complete DOT number normally 10-12 digits is only engraved on one side of the tire, the other side will be lacking date coding.  Why is it so important to know the date? Because tires age out and must be replaced based not only on wear but their age as well. Depending on the source you look at recommended age required replacement ranges 6-10 years. Dry rotted and cracked tread and sidewalls are one sign a tire is starting to age out. There is no federal law requiring tire replacement at a specified age, but most shops will refuse to mount a tire that is 10 or more years old and will start to recommend your tires be replaced if there are signs of dry rotting or they are 6 plus years old.  The environment and care tires are subjected to also will influence their useful life span, I have replaced tires less than three years old due to sever dry rotting. Remember to check the date code on new tires you are purchasing  because they may have been on the rack at the tire store for years before they are ever sold. Always purchase tires with the latest date code possible. Have your tires checked regularly at oil change time for condition issues of any kind and replace them before they leave you stranded or cause a more serious problem.