My bike has done 10,500km or 6,500miles and is 18 months old.
Lately it seems the front forks has softened up, they don't seem to have much resistance in compression.
There is no oil leakage, is this possible or am I just imagining it?

Are they the stock springs or aftermarket?

I'm assuming stock springs, which are notorious for being soft anyway. Chances are, your riding has improved over the past 18 months on the bike, and you're now realizing how soft the front end has been the entire time.

I've also been told that after 2000km's your fork oil is likely sludge, so might be time for a fork oil change.

I've noticed the front end of my 450 is not as good as when I rebuilt it and I have almost 26,000km's on it now since the rebuild. Next service will be over the Chrissy period and I'll be doing fork oil at the same time.

Incidentally I swapped out the stock springs for Ikon progressive springs and the difference is remarkable...

These forks are garbage after 0 miles. You got 6k ... that's 6K more than they were intended to be used ...

Cool.
Buddha.

What The Buddha said. Replacing the front springs is trivially easy, and can be done without removing the forks from the bike.

With that said, oil breaks down. For road bikes, we usually advise changing the fork oil every 2 years or so.

Two things happen with the oil:

1. It fills with metal particulate. This tends to make the suspension stiff when cold.
2. The oil sheers and thins out. This tends to make the suspension soft when warm.