Potency of the age-reversing water is wildly uneven. Only a few drops revert Hex back ten or twenty years, but a splash of water reverts Ben back only six years, and immersion in gallons of the water only reverts Grandpa Max back about fifty years.
Hector says that the effects of the fountain of youth water wear off after awhile, returning those affected back to the previous ages. Hector has been drinking it for 400 year, but when Ben and Max revert to the proper ages after destroying the fountain, Hector distantly waves them goodbye, still a young man and not the dead-of-old-age 400-year-old crone he should be. (Hex, having drunk more later than Max and Ben, is still a toddler.)