On Marlin-branded Model 60 rifles, the first two digits of the serial number indicate the year of manufacture. For example, your serial number starts with 09...... the 09 is significant since it is subtracted from the year 2000 to give the year of manufacture, 1991.
This method is valid only from 1971 through 1999. Prior to 1971, the first two digits indicated the year of manufacture. For example, a rifle with a serial number starting with 70... would have been made in 1970. Starting in 2000, this same old method has returned; the first two digits once more indicate the year of manufacture directly, with no subtraction required.
And, from 1962 to 1968 Marlin used letter codes on the barrel, first letter was the month and the second letter was the year of manufacture.
If that's not confusing enough, starting in 2000, they went back to using the year as the first two digits of the serial number so in another year or so, you won't be able to tell if your serial number 09 was built in 1991 (2000 minus 9) or if it was built in 2009.
Dan R