Coolants should be changed every two years because the corrosion inhibitors that they contain, deplete and become ineffective. The resulting circulating corrosion products are particularly abrasive to water pump seals and the low activity levels of old inhibitors can allow pitting of aluminum radiator surfaces and general corrosion of metals in contact with the coolant.
If you test the coolant for alkalinity, you could be assured it's still safe.
I just had the coolant tested in my 7 year old car (99,000 miles) and it was still good. But I knew the history. It's long term coolant.
You can get a test strip to check the ph from an auto parts store, if you know what kind of coolant is in your bike. Or you can test the alkalinity of the coolant with a multimeter.
Put the meter on a low DC voltage scale. Put the + probe on the battery negative post and the - probe into the cool radiator filler opening, under the coolant level, but not touching any metal. (I could have the polarities backwards, but it doesn't matter.)
An good reading is .1 volts or less. Anything above .3 to .4 volts is time to flush and renew the coolant.
But to be truthful, our bikes hold so little coolant that it's not worth messing around with. Dump it and replace it.