When Atrus was born, Gehn was nineteen. But when Atrus is around seventeen or eighteen, Gehn says he's been practicing the Art for thirty years, since he was four. But that would make him only thirty four old when he is actually around thirty six or thirty seven years old.
Gehn may have just rounded off...I know many people who do this...When talking about eleven years, they say ten years..Like "He did that ten years ago." I wouldn't actually count it as a slip. Imperfection adds to character development.
This really cant be counted as a "Slip Up" a Dn'i year or "hahr" is split in to 10 equal segments called "vI-lee-tee". One "vI-lee" is roughly equivalent to one Earth month therefore a Dn'i year is only 10 months (roughly), as Gehn has lived in both Dn'i and on the surface it is difficult to know exactly what he meant by "years".