As mentioned in several posts now, if your livestock is fine after 6 years of using it you're probably ok. On the other hand, without comparing it to a known value or another hydrometer, you'll never really know how accurate (or not) it is. Most salt mixes usually tell you on the packaging the ratio of salt to water for a given salinity value. That might be a good way to check it out without having to go out & get a refractometer or glass floating hydrometer.
I've used all three types & the floating glass hydrometer is actually quite accurate. Mine had a thermometer built in, since the floaters are calibrated at a certain temperature, so can be off a point or two if the solution you measure is significantly higher/lower than the cal temp. Most refractometers offered these days have automatic temperature compensation.
Results of measuring the same water with a floating glass hydrometer & 2 swingarms. I also used a refractometer & its reading was the same as the floating glass hydrometer @ just above 1.024.
Glass
Coralife
IO Deep Six
As is evident, both swingarms were reading lower with the Coralife low by more than 2 points. If I had been using this to mix & measure water, the solution would end up around the 1.030 mark which is probably not good long term for livestock. While I don't use the swingarms these days, I still have them & used a permanent marker to correct the scale at 1.025 in accordance with the refractometer reading. Since they work on the principle of a weighted fulcrum, there's not much reason for them to lose whatever calibration they have. Only way is for the weight to be moved along the arm or contamination caked on the arm that would change the weight. Temperature might also be a factor. Other than that, once you've confirmed it's reading with a known value, it should be fine to use as long as you keep it clean.