Struts/shocks usually lose their dampening ability slowly.....so slow in fact you don't notice it happening. There's no set mileage for this to happen. But you usually can check the struts by bouncing the suspension.
If the fronts bounce even one time, they're probably weak. Normally you can push down and they slowly return. You got plenty of miles out of those struts. A new set is going to feel much better.