Most fork oil needs changing between 20-30k miles...a good indicator on when to do this is when you can see the seals are leaking...more obvious on USD's than normal forks.
For most forks, age doesn't make any difference in my experience...best to get the correct manual for the bike and confirm if there are any yearly intervals for doing them.
PS: Make sure you change the bushes when changing the seals...as it's usually play in the bushes that causes the seals to fail in the first place...for example genuine set of bushes and seals for a GSXR K series is around £65 for a set of forks.
As for the rear shock...if it's like a pogo stick...replace it...but pref with an aftermarket one (Ohlins/WP/Maxton) and the bike will be transformed.
MH Racing in Corsham is the best person to see about it all...used him to do my shocks over several years now and he's always spot on
