Wow! This was a very interesting question, and it took a quite a bit of thought, and the answer is rather disappointing for me. First of all, I have been buying guns since around 1964. Unfortunately I have always been searching for that gun that I wanted the most, so it meant trading many many guns off, because I didn't have the cash for the next purchase so something got sold or traded to get it.
So, the gun that I have personally owned the longest would be a Volquartsen custom Ruger MKII Match Carry that I bought new in March of 2004. Now, I have a Winchester model 67 that belonged to my Grandpa which he bought around 1934 I think, but I inherited it in 1989, so I guess that makes it the oldest and the Volquartsen the second oldest. Anyway, I have owned many very nice guns in my lifetime. I just wish I hadn't traded many of them off. Oh well, life is an adventure and no one owns a gun forever ya know.