I'm reporting the result of a study with specified target cohorts (e.g., people born in 1985, in 1990, etc.). There are participants who don't belong to these target groups (e.g., ones born in 1986) but have participated in the study anyway. Can I refer to them as outsiders ?
I believe the word you're looking for here is Outlier.
Dictionary.com gives two relevant definitions:
something that lies outside the main body or group that it is a part of;
an observation that is well outside of the expected range of values in a study or experiment;
The students you are looking at are a part of a class, but due to their year of birth, aren't in the main sub-group (those born in 1985). If you put their birthdays on a chart, you would see the bulk of students clustered around 1985, with only a few outliers around 1986.