Federal and state law prohibit discrimination based on age. This only applies, however, to people who are 40 years old or older. In other words, it is better expressed as older work discrimination than age discrimination, because younger people are not protected if an employer discriminates because they are perceived as too young.
The cut off is 40, and has not changed in the decades since the age discrimination laws were enacted. In practice, as the average age of the workforce has increased, examples of actionable discrimination tend to (though not always) involve workers who are fifty or older.
There is also often an overlap between age discrimination and disability or perceived disability discrimination. For example, if you are older and have a health condition, your employer may start to think about your age differently than before the health issues were disclosed.
Learn more here about discrimination in Massachusetts.