I am not 100% percent sure the oldest single active volcano is but there a plenty of volcanoes that take several hundred thousands of years to form (think the active volcanoes in Hawaii, Mount Kilimanjaro, Mount Fuji). The magma that underlies places like Hawaii has been there for million years. For Hawaii that would be maybe close to 100 million years. However because of plate tectonics moving the Earth’s crust this magma forms many different volcanoes over time (for example the different Hawaiian islands). Hopefully that was useful!