In response to Alaska congressman Don Young's statement that "My father had a ranch; we used to have 50-60 wetbacks to pick tomatoes,” Dave Weigel has tweeted:
Re: Don Young, look, old people sometimes use outdated racist terms. I've met old people who voted for Obama and called him "colored."
— daveweigel (@daveweigel) March 29, 2013
Weigel has also compared this to Harry Reid's 2010 comment that Barack Obama speaks "with no Negro dialect." The response has generally been that "Negro" and "colored" aren't slurs, whereas "wetback" has always been a slur.
And that's true. It's always been a slur -- but white America hasn't always recognized that.
As Wikipedia notes, the first appearance of "wetback" in The New York Times was in a 1920 article:

The U.S. started a border security program in 1953 called Operation Wetback. That was the year Don Young turned 20. A Google News archive search reveals that the word "wetback" showed up in quite a few newspaper headlines in the 1950s, '60s, and early '70s:
* "Wetback Net Used Lightly; Aliens Are Useful" --Milwaukee Journal, June 29, 1950
* "'Wetback' Entry Reasons Told" --Deseret News, July 15, 1954
* "Wetback Influx Shows Decrease" --New York Times, July 6, 1958
* "Wetback Flow Cut by US-Mexico Pact" -- Milwaukee Journal, April 19, 1960
* "The 'Wetback' Problem Has More Than Just One Side" -- Los Angeles Times, April 24, 1970
* "Wetback Problem" -- Milwaukee Sentinel, December 31, 1971
But public officials were getting flak for saying "wetback" decades ago -- here's one in 1989, here's one a year later. And, well, Young lives in the same society as the rest of us, doesn't he? He has no excuse for being oblivious to the fact that the word is now considered offensive.
So, yes,, I take into account the fact that some people used the word freely well into Don Young's adulthood. But that's still no excuse.