Age is almost all cases matters. If a fighter has 150 fights, or over 35, or takes significant time off from the ring, their perfomace is not as good as it was when they were 30. In almost all cases this is true. Hopkins is a rare exception of being of being an elite level older fighter, There are few like him. Factoring who you beat is often as important as when you beat them, and how you beat them. I find some historians often ignore age and in favor of the fighters name and reputation. That and ignore the struggles and losses. Older historians just don't have the tools of viewing that we have today. In 20 years I predict many of the fighters held that regard today will be viewed differently. Like I said we have a lot more tools to view them.
Usually, but there are degrees. Some are disasters, shells, others have slipped a bit, some even improve, though that is rarer. If a fighter boxes well he's a good boxer, regardless of age.