Can you push a finger through the substrate all over the tank? The only time I went through the substrate and completely vacuumed it out was due to some sort of fungal/root/decomposition/algal growth that bound the substrate so tightly I couldn't push my finger through it. Weird experience and haven't seen it since.
Mostly I leave the bottom alone but keep the top part clean looking. When I change tanks I have the old tank partly filled so when I transfer the substrate some mulm is rinsed off but it is still very 'dirty'. The 100 gallon tank's substrate was at least 5 years old when I transferred it to the 180 gallon tank and there wasn't as much mulm as I thought and that old substrate was sweet smelling like good earth.
I haven't rinsed old substrate clean since 2000.
If you have the rescape itch then do take out all the plants and fish and any other critters, mix up the substrate and drain all the water out, rescape and plant, fill part way and dump that water as you vacuum the surface then slowly refill so the water stays clear. That would remove a lot of the mulm but not all of it.
That written, if you don't have plant growth or algal issues then you are probably just fine. I wouldn't call that old substrate yet.