Kendall and Kylie Jenner may be underage but they apparently already know how to party.

According to reports, the teenage “Keeping Up with the Kardashians” stars, Kendall Jenner, 18, and Kylie Jenner, 16, were both spotted drinking what appeared to be alcoholic beverages during their big sister Khloe Kardashian’s birthday bash over the weekend.

Entertainmentwise is reporting that the underage duo was photographed sipping out of champagne and wine glasses during Kardashian’s 30th birthday aboard a yacht in New York City’s Pier 61 Friday. “Kylie enjoyed a quick glug from her large wine class, apparently not concerned about being spotted by her fellow party goers,” reads the report, adding, “Kendall was similarly nonplussed as she drank from a Champagne flute."

While the youngest Jenner's have yet to speak out about their alleged alcohol intake, Khloe Kardashian seemed unfazed by rumors of her half-sister’s underage partying. “Thank you for the birthday wishes!!” Kardashian tweeted Saturday. “You helped make me day incredible!! Bless you all J I love you! #Unforgettable.”

The reality-TV star’s party, also attended by fellow “KUTWK” stars, sisters Kim and Kourtney Kardashian, and mom Kris Jenner, follows Kardashian’s first celebratory bash at Don Coqui nightclub in New York’s Bronx Thursday. E! News reports that Kardashian partied the night away beside 50 guests including her family and new boyfriend, rapper French Montana.

According to TMZ, Montana, 29, spent a pretty penny on his reality-star beau’s big day, reportedly gifting her a $50,000 Jeep Wrangler Sahara, a $25k 18k gold ring, and a $10k diamond grill.

Kardashian is set to celebrate her birthday for a third time at Tao Nightclub in Las Vegas on July 4th. Tickets for the event are retailing $20 for women, $40 for men. Guests must be 21 years of age of older to attend.