HO12 is the water from the Fountain Of Youth that is 1 Hydrogen atom and 12 Oxygen atoms. It also contains high levels of carbon. The rare element (found only in the aforementioned fountain) is capable of literally rejuvenating anyone who drinks or comes in contact with it, but only temporarily. For reasons unknown, the fountain's protector, Hector, had been keeping a dunk booth full of it at an amusement park, in which Max was dunked into and which was later splashed on Ben when the dunk booth was smashed.

How many years are shaved off apparently depends on how much water you take in at a time. Max was dunked in and stayed in for a short time and was rejuvenated by 50 years. Ben, on the other hand, only got splashed for a second and his age went down only 6 years. Because the Omnitrix was designed so that the aliens inside would age along with its host, Ben's alien forms were affected by the properties of the water as well. The fountain was destroyed by Baby Heatblast, so HO12, for the most part, no longer exists.