This comes up all the time:
Simple answer 1985-1990
A tractors age is date it was sold, so it really doesn't tell you the "real age of the tractor, because it could have been sitting on a dealers lot for years before it was sold.
Only way is to contact a Kubota dealer with the serial Number of the tractor, they might be able to tell you.
All of this really doesn't matter because a L2250 is the same throughout it's production, it will have some variations in parts, But armed with the serial number you should be able to get the right parts.