Tyres - Age
To check the age of the tyres, refer to the 3 or 4 numbers stamped in an oval on the side wall of the tyre. For a tyre manufactured after the year 2000 (4 digit identifier) the first two digits refer to the week number (i.e. Week 50 = December) and the last two refer to the year.
For tires manufactured before the year 2000: The date of manufacture is the last three digits of the code. The first two digits refer to the week within that year. For example, if the last 3 digits are 022, it means that the tire was manufactured in the second week of the year, and the year is the second year of the decade. This is where it gets confusing -- there's no universal identifier that signifies which decade, so in this example the tire could have been manufactured in 1982 or 1992. Some tires do have a small triangle following the DOT code to indicate the 1990s
Age to replace
Prevailing wisdom is to change your tyres at least every 10 years, regardless of the amount of wear present. Some tyres will age better than others.
LEC write up with details from Lotusflasherman