What is the verb for criticizing someone (especially juniors) in an annoying way?

nagging
adjective
(of a person) constantly harassing someone to do something.

"Mom don't start nagging again, please."

I like harping, meaning to repeat the same criticism over and over again to the annoyance of its target or recipient.

Mom, why must you keep harping on the length of my hair?

"Harping" is perhaps a tad disrespectful, considering you are talking to your mom, but you could soften the word nonverbally in a variety of ways.

Condescend or condescending (verb / adjective)

Having or showing an attitude of patronizing superiority.

Ref: Google Dictionary

Your original example:

"You young people! Really, you do not respect anything. I do not know what has happened to you. You should have at least cut your hair".
"Mom don't start inquiring again, please. What is wrong with my hair?"

You could re-write like so:

You young people! Really, you do not respect anything. I do not know what has happened to you. You should have at least cut your hair.

Mom don't condescend again, please. What is wrong with my hair?

or (more typically):

You young people! Really, you do not respect anything. I do not know what has happened to you. You should have at least cut your hair.

Mom don't be so condescending, please. What is wrong with my hair?