Ilvermorny was started in the stone house on Mount Greylock in Massachusetts by witch Isolt Sayre and No-Maj James Steward as a way of educating their magical adopted boys Chadwick and Webster Boot (Pm).
As word spread, they were soon joined by other local magical children from members of the nearby Narragansett and Wampanoag tribes. James Steward became a wand-maker for the children as they turned eleven, using various woods including prickly ash, and wand cores including Horned Serpent horn, Wampus hair, Snallygaster heartstring and Jackalope antler (Pm).
We know Ilvermorny was founded when Chadwick Boot turned eleven, and that Chadwick was sixteen at the attack on Ilvermorny c. 1635. The school could not have been founded before 1629, and most likely around 1635.
From the Web
History of Ilvermorny School on Pottermore