молоко на губах не обсохло (moloko na gubakh ne obsokhlo)

Literally: Milk has not dried on the lips

Example: “Он учит меня водить, а у самого молоко на губах не обсохло.” — He's teaching me to drive, and he's still wet behind the ears.

English equivalent: Wet behind the ears

Origin: The milk is the mother's milk, as Russian Wiktionary notes: someone on whose lips it has not yet dried is barely out of babyhood. It is said dismissively of a young person who acts more experienced than they are. Italian has the same image in avere ancora il latte sulle labbra.