I think the issue isn't really one of age, but rather one of whether or not the step was of high quality in the first place.
Not that I claim to be an expert on step quality, but it seems to me that one issue is if the step is stable and will not tip over or "come apart". This is probably more of an issue the higher the step goes. The other issue may be if it the top still has "traction" or if it will be slippery on your foot. (check your shoes for traction too!)
To test it, I would do some practice movements--maybe some turn steps or quarter hop turns, not pure forward movements--on it with all the risers and be prepared for it to come apart. Just how you'll do that I don't know! Also, maybe you could ask the gym why they are getting rid of them. Maybe that just don't look nice anymore.
I have a Reebok step that is about 10 years old and still works perfectly fine. (But for the record, someone here once said that this step is not nearly as cushiony as "the step").
Happy stepping! JEanne