mileage varies wildly - 60,000 - 150,000 depending on road conditions, driver perceptibility, etc.
they can degrade substantially without failing/leaking.
how do the struts seem now?
how does the car bounce? when you hit bumps/holes while going over slight turns can you feel the car kind of "shift sideways"? any noises? leakage at the strut cylinder?
if there's any symptoms then replacement would be a normal consideration.
if no symptoms and they ride great then maybe you hope they last the life of the vehicle? that wouldn't be the normal approach but you'll definitely get symptoms if something starts to degrade substantially or fail, so you'll have a warning of some sort.
if you think you'll be replacing them before you get rid of the car, then doing it sooner rather than later gives you more return on the new parts.
new struts will prolong tire life and protect top mounts and springs, end links, etc, which take more of a beating if the struts are weak. i've seen all of those components fail on 2000+ Subarus with old/weak/high mileage struts.