Software doesn't age, it doesn't make sense for your computer to become slower as it becomes older. (some) Software just becomes more shitty and bloated with every release, which is what you're experiencing.
I think there's room for an exception here: operating systems or other software that handles a large number of files could bog down with use as the number and size of files grow with time.
Doesn't help with the bloated web and local webapps, though. Also, you'll need to choose from a set of desktop environments that were made with lower resource usage in mind. Also don't forget that while linux is often faster, a slow drive is still a slow drive and it can help only so much if you keep your OS and heavyweight software on a HDD.