As David Letterman and Howard Stern can attest, a 30-year anniversary celebration is a ripe opportunity to cringe at geeky haircuts. Letterman marked three decades in late-night TV on Wednesday by bringing radio host Stern, one of his favorite guests, onto his “Late Show.” They congratulated each other on age improving their looks. Then Letterman proved it, displaying a picture of them from when Stern first appeared on the show in 1984. Letterman had a bushy head of hair. Stern had a mustache. Letterman said about Stern’s photo: “It looks like a guy who was on to talk about a career of writing bad checks.” Stern advised burning the picture but recalled how grateful he had been for the invite. “Back then, the fact that you asked me to be on a national television show was mind-blowing to me,” he said. Letterman also brought some of his longtime employees onstage to present the “Top Ten Things Staffers Would Like to Say to Dave on his 30th Anniversary in Late Night.” One of the cleanest, and most succinct, entries came from veteran executive producer Rob Burnett: “You’re incompetent.”