There is a code moulded into the sidewall where the other tyre info such as load rating is marked.
Pretty simple to interpret: examples follow:
Code 3712 = a tyre made in the 37th week or 2012.
Code 0915 = a tyre made in week 9 of 2015.
Do some research and you will find that the various manufacturers will say that tyres of 5 or 6 years of age shouldnt be used due to the effects of ageing. And add in salt water getting into steel belts and that probbaly accelerates deterioration of boat trailer tyres. Number of KM's travelled has nothing to do with it.
This quote from Bridgestone Australia:
How old is too old?
This is a subject of much debate within the tyre industry and no tyre expert can tell exactly how long a tyre will last. However, on the results of experience many tyre companies, including Bridgestone, warrant their tyres against manufacturing and material defects for five years from the date of manufacture. Based on their understanding a number of vehicle manufacturers are now advising against the use of tyres that are more than six years old due to the effects of ageing.