Sure, that's not the way to figure out life expectancy since you're only seeing people who died. For example, a lot of the original wild men of the 1950s aren't dead yet (Chuck Berry is 82 and Little Richard and Jerry Lee Lewis are in their mid-70s.) And Les Paul, who sort of invented the electric guitar, died recently at 94.
Still, 60 or thereabouts is what I keep seeing in ex-rock star obituaries.
In general, rock stars tend to be high energy people. For example, a thoroughly hashed-up and beered-up Joe Strummer of The Clash ran marathons in 1981, 1982, and 1983, claiming a best time of 3:20, which is pretty funny considering the shambolic state of The Clash from late 1979 onward.
Kurt Cobain is close to the exception that proves the rule by being a rock star who probably would have been sickly even without all the drugs. Neil Young suffered from epilepsy when he was in his 20s, but it has disappeared, and his iron constitution has reasserted itself (his mother was a terrific athlete and his father was Canada's top hockey writer). For Bruce Springsteen, who appears to be a generally all around above average individual, I'd set the over-under line at, say, age 87.