When any legendary performer who’s climbing up into his years comes anywhere close to Little Rock, I dial up his age, add about 10 years for hard living and road wear and, if that puts him anywhere near 80, which is the upper reaches of an average American male’s life expectancy, I usually drop what I’m doing and catch a show. Apply that system on Friday and you’ll feel an extra bit of urgency when you go hunting tickets. Willie Nelson, who was probably drinking and smoking weed when my father was but a gleam in my grandfather’s eye, is 75, and B.B. King, who’s surely played more gigs (an estimated 15,000) than anyone ever, is 82.