sippy cup
Juvenile or immature.  From the popular children's drinking device 'a sippy cup', made with a covered lid with narrow built-in sipping slot to prevent spillage.  Toddlers drink the beverage inside via the narrow slot, and the drink is almost impossible for them to spill because of the cup's design.
"God, look at those girls over there wearing Elmo-t-shirts at the mall.  What, are they six or something?"
"Yeah, like that is so sippy cup already."

or

"What was it with rappers all wearing pacifiers and baggy toddler clothes a few years ago?"
"Yeah! Like was that sippy cup, or what?"