TV & VIDEO - Aug. 11, 1989

A two-hour prime-time special on Sept. 24 will mark the “Saturday Night Live 15th Anniversary.” The broadcast will include clips from the late-night satirical show. . . . NBC’s “Later With Bob Costas” will celebrate its first year on the air with a 90-minute special, “One Year Later,” airing in the wee small hours of next Friday night (1:30-3 a.m.). . . . Tonight’s edition of “Friday Night Videos” (1:30 a.m. on NBC) turns out to be a surprise birthday party for Malcolm-Jamal Warner of “The Cosby Show,” who arrived at the taping the other day thinking only that he was to host the show with Ahmad Rashad. Warner turns 19 next Friday.