If age seems to be one of Mose Allison’s lyrical preoccupations, it’s also one of his distinctions. The 80-year-old singer/pianist was born in Mississippi at a time when jazz and blues were more or less interchangeable—and in his music, they still are, along with R&B and even postwar pop crooning. Though Allison says his genre-blurring has made it difficult to maintain a steady audience, those who’ve remained loyal include Van Morrison, Bonnie Raitt, Leon Russell, and the Who—all of whom have recorded some of Allison’s stellar compositions. But good as those covers are, Allison’s songwriting is best experienced through the filter of the maestro’s furiously rhythmic, greased-lightning piano technique and his wise but youthful voice. He makes the advancing years seem all but irrelevant, save for the years of study evidenced in the musical encyclopedia that Allison squeezes into every performance. Tonight’s show closes a three-night run at Blues Alley. THE MOSE ALLISON TRIO PERFORMS AT 8 AND 10 P.M. AT BLUES ALLEY, 1073 WISCONSIN AVE. NW. $25. (202) 337-4141.