As far as their writing ability, i don't think anyone here would argue that it isn't what it once was. That's the part that i have the most trouble accepting, because i can't find an intuitive reason why musicians (or writers) should lose their ability to write profoundly as they get older. In Rush's case, i think most people on this board will agree that it started happening in the mid 90's, if not sooner. They were still in their early-mid 40's at that point.
As you age, you lose your boldness, you become more aware of - and afraid of - ridiculousness. The young are more focused, more consumed. Many writers - you could look at Longfellow, or Hemingway, or Faulkner, or Dickens - have their most acclaimed literary triumphs early to mid career, and then gently fade, producing less ambitious but nevertheless polished works.
Or, they kill themselves.
After a band, or musician, peaks, there is often a fairly predictable pattern wherein the performer grows increasingly bombastic, then selects an alternative genre to dabble in, then does "back to the roots", and then a final hurrah. Close curtain.
I sometimes think about these things in my own writing, which is occasionally for profit, typically for personal or fraternal pleasure, and always for fun. I wrote more ambitiously, more fearlessly 20 years ago. Now my writing has become measured and more pithy, just as I am as a human being.
Did you know that Emily Dickinson wrote the majority of her poems during a 5 year period while in her 30's?