“Ok boomer”.    slang term 

Created by Generation Z as a clap back to older people when something critical is said to/about younger generations. 

[also see “Boomer”: adjective
Originates from Baby Boomer, but can be used to describe an individual of any age who acts like they are a million years old and/or is stuck in the ’50s. Most likely problematic.] 

Ex. “Kids these days are so lazy, everyone is always on their phones, when I was young I had to walk 40 miles if I wanted to call someone.” 

“Lol ok boomer.”