Mr M, dont you rate albums like The Wall/Animals - Pink Floyd, Songs From The Wood - Jethro Tull, Songs In The Key Of Life - Stevie Wonder, Beautiful Noise - Neil Diamond, Rust Never Sleeps - Neil Young, Wired - Jeff Beck
these artists had been around for 10 years I think
It's funny you should mention "Songs From The Wood" Kev - one of my favourite albums ever - released nine years (OK, I know I'm hairsplitting!) after Tull's debut. It kick started a consecutive three album run of superb quality (i.e. followed up by "Heavy Horses" then "Stormwatch") which did finally cross that ten year boundary line, and I was tempted to cite it as another exception to the rule. But as obs has reiterated; overall there are relatively few instances. I'd go along with your Stevie Wonder and Neil Young examples but I'm struggling a bit beyond those. Never grasped the whole Floyd thing (to my son's perpetual exasperation) so can't really comment on that I'm afraid, but by and large I still say obs's rule of thumb tends to hold up. Even if an act does hit a purple patch a decade or more down the line, it seems to usually be a flash rather than shifting up to a consistently sustained better level.