Service by years or mileage, which takes precedence?

It is a good idea to go ahead and change all the fluids.

I'd ignore the diff and brake fluids (unless the brake fluid reservoir cap was left off). But DO drain and refill the ATF (3.5 qts). The plugs may be still ok, but these OE NGK platinums tend to blow off their fragile anodes, so I'd replace with any modern iridium equivalent just to be sure. Thereafter change the ATF biannually, check the t-belt for cracks and air filter annually. Leave the diff alone forever. In general, cold starts and age kills these chariots...NOT mileage. Only lifespans of tires...and possibly Subie wheel bearings...are correlated with distance traveled. Potholes eat bushings and links, so judge independently too.

Re the dealer's comment: only highly volatile fluids will thicken with age. That means gasoline, not lubricating oils.