LOS ANGELES (AP) — Neil Young does a great Jimmy Fallon impersonation. Or was it the other way around? It was hard to tell at times last night, as Fallon had the veteran rock star on “The Tonight Show.” After the first commercial break, Young was introduced — and the TV audience saw a performer on a stool, wearing a large hat, a harmonica around his neck and strumming a guitar. It was Fallon, performing “Old Man.” Eventually, Young joined Fallon and they finished the tune together. Young also performed with The Roots — the show’s house band — to close out the show.