The original series of H60's used a flexible almost plastic tubing (still very stiff) so coolant evaporation as such is almost negligible. However, what you get from constant heat and fluid is a breakdown on the molecular level, such as H²O ceases to be and becomes HO or H+H+O, which will eventually drop fluid levels as gas levels rise. Other components might lose much of their fluid nature and get thicker and more sludge like.
But this generally takes years of usage, a good starting point being 5yrs. After that, it's just a waiting game, if the cooler still works, great, but it's going to start getting gradually worse at some point until fluid levels are so low, the pump can't effectively pressurize the fluid volume and your temps go up drastically.
So yeah, it's time to start thinking about replacement, but you've got time to wait and watch for a good sale/deal, it's not failed so no rush.