Prosecutors said on Saturday that the victims were aged 14 to 20 years, and that a nine-year-old was among the wounded.
"A group of heavily armed men arrived in two minivans. At least 10 men burst into the party," Carlos Gonzalez, a spokesman for state prosecutors, told the Reforma newspaper.
The partygoers had gathered for a 15-year-old boy's birthday party on Friday.
"The victims were in the back yard of the house having a party when hooded men, in dark uniforms and with rifles, arrived in several vans, broke in and began shooting indiscriminately at those inside," a police official said.
The attackers escaped, and police said they had no immediate information on any suspects or possible. Juarez is the epicenter of Mexico's battle against drug gangs, and it's the world's most deadliest cities.
Seventy bullet casings from assault weapons typically used by drug gangs were found at the scene.
Local media, which showed images of bodies strewn among cars parked in the garage of the house, said 15 people were killed and 18 wounded, but that toll could not be independently verified.
The massacre occurred in a residential district known as Horizontes del Sur, which has experienced a shocking rate of violence.