My first “mutant experience” came from the pages of X-Men #1 (1963), when Professor Charles Xavier gathered the original X-team of Cyclops, Marvel Girl/Jean Grey, Angel, Iceman and Beast. By default, Xavier being the oldest person in the team made him the oldest/first mutant ... or so I thought at the time.
Since those Stan Lee-Jack Kirby days, the various X-teams’ rosters have evolved and so have their enemies, resulting in the introduction/retconning of many characters who are way older than Xavier (which makes the transition from James McAvoy to Patrick Stewart much more acceptable).