Time and miles are both factors, but miles have relatively more influence.

If a is strut bad, replace it, and remove the top strut mount and clean and inspect it thoroughly. If the chassis has less than 100K miles the strut mounts are probably okay.

The struts can be replaced singly. They don't have to replaced in pairs. It's not uncommon for one strut to fail completely as evidenced by lots of oil loss while the other is okay. These high gas pressure "de Carbon" type struts maintain their damping characteristics throughout their life and then usually fail completely unlike conventional hydraulic shocks that slowly degrade with use.

Always replace the strut dust boots when the front struts are replaced as they will likely disintegrate before the strut fails.

If no other problems are observed and the car drives okay, leave everything else alone.

Duke