replacing shocks

[w9det]

When should you change shocks?
My 2001 Safari 27 has 55,000 miles on it and I would think age is on my side. The shocks look good, a little rust but no leakage. Shocks on cars etc. seem to last forever now in instead of the 20-30K of years gone by.
Am I wasting my time by changing them now, anyone know the bench mark for shocks now. 50k, 75k, 100K. The trailer tracks nicely and the tires wear evenly.