Student or Learner
What could be said of a person who judges people on basis of whether their interests are kiddish or mature. For example, someone at age 30 likes playing video games and he is ridiculed saying "Wow, you play video games at 30, are u a kid". What would you call this person or this judging.
A one-sided judgement?
Not a teacher.
ageist (or agist).
They should, of course, say "Wow, you play video games at 30, are you a kid?"
I would say they were being judgmental.
Remember - if you don't use correct capitalisation, punctuation and spacing, anything you write will be incorrect.
"What would you call this person or this judging."
I understood that the OP already knows that this is judgemental.