Partly true, but there are plenty of bands who make successful music into their 40's and beyond. I'm not a fan of either but U2 and The Red Hot Chilly Pepper have both enjoyed some of their biggest critical and commercial success in their 40's.
Paul Weller's last album (22 Dreams), made on the cusp of his 50th, was possibly the most acclaimed album of his career. It sold pretty well too. Some of Bob Dylan's recent albums have been considered to be among his best along with those by Bruce Springroll.
So I don't think we can really put down Morrissey's disappointing decade to the fact that he was over 40...
I agree with you. I think older artists are also wonderful, but in a different way. I'm not sure how I would distinguish them, exactly. Probably just a vague thing like spirit or energy. The superb "Stanley Road" is much more polished, assured, and well-crafted than "Setting Sons", but there's no contest as to which I prefer.
Perhaps the difference is that, later in their careers, musicians are on a career track. I don't mean they are money-grubbing careerists, just that, beyond a certain point, they're a brand, a mini-industry unto themselves. Pop music needs inspiration far more than technical skill.
For example, you mentioned U2. I distinctly remember them crossing a line in the late Eighties from talented amateurs to actual musicians. In interviews they'd talk about learning all the chords on the guitar and how to write "proper" songs. Certainly they continued making good music after, say, 1987, but their most exciting songs are the ones early in their career when they were just mucking about with sound. It was when they embraced the whole rock and blues traditions, and tried to become actual musicians (only Larry was a trained player), that they became both better and worse. Better from the standpoint of craftsmanship, worse in the way the music began to lack spirit and (pardon me) pop. You can see this on the much-maligned "Rattle and Hum", which is the "professionalized" version of "The Joshua Tree". They only regained their form (briefly) when they began experimenting with electronic music, that is, they went back to being amateurs for the space of an album or two.
You can more or less discern the same arc in New Order. When Barney tried to write proper lyrics for the last few albums, the songs nosedived. Zonked out of his mind on drugs, making up gibberish as he went along, he was a much better songwriter.
And to bring it back around to Morrissey, well, I've gone on the record many times with my opinion that his voice has never sounded better.
Years of Refusal has some of his most technically strong work ever. Yet I still love-- and often prefer-- the older "substandard" vocals, even as far back as the early bootlegs from The Smiths' first shows.