There are two answers to this question
1) the oldest item of clothing I have is, I believe, the red 100% polyester shirt above. It's from the 70s, though I didn't purchase it until 1997 or 1998 from a thrift clothing store in Cambridge, Mass call The Garment District.
I also wore this shirt during my audition in 2002 for the HBO Comedy Festival. A New York Times reporter took the shot of me at The Comedy Garden at Madison Square Garden. You can see it on my photo gallery at baratunde.com
2) the item of clothing i've had for the longest time is the t-shirt pictured above. It says Ankobia, which is the name of a Pan-Afrikan rites of passage program I was part of in the early 90s. I've had that shirt since around 1991.