This may not apply to certain older models, but generally speaking, you should be able to determine the model year from the serial number. Typically, the number is scribed into the kayak...right side...close to the stern.
The last 4 digits of the series = date stamp.
For example, WKYABC12A909: (last 4 digits bolded for emphasis)
"A" (month) =... Jan.
"09" (model year) = 2009
Most, but not all boats, have a recessed area here. Your serial number will always be in this area.