Because how old someone is is relative to the current time. And that’s the wording that the commenter used: People who are x years, y months, and z days old. The next day those same people will be a day older.
Say the discrimination was about people born on Dec 20 instead of April, in that case they (where I am) are currently X years, 11 months, and 30 days old, and tomorrow is their birthday.
I just realized that they did calculate it the wrong direction though, the 4/20 peeps are 3 months and 30 or 29 days old today (not sure on that) today.