According to i-Boats, "Nothing in the Mercury Mariner Outboard engine model numbers or serial numbers provides year information directly.
The first serial number sold in a year becomes the beginning serial number. Serial numbers between that year's beginning serial number and the next fall into the same year. Search for your engine horsepower on the Mercury Mariner serial number year chart located here to find the beginning serial number for the range that includes your engine." You other option is to go to any authorized Merc dealer and hey should be able to loo it up.

Call mercury outboard they can tell you the answer to your question