It is 6+ months for the upright position. This is from their FAQ section on their website.
The City Mini 2, City Mini GT2, and Summit X3 strollers have an increased recline angle to a near-flat position to accommodate newborns. That means you can safely lay a baby in the fully reclined seat of these strollers without the use of a car seat adapter or bassinet. While using a Summit X3, never run or jog with your seat fully reclined. Also, please keep in mind that babies should have ample neck support when placed in the reclined seat of a stroller—you can use any number of neck supports sold at baby stores.