I was considering buying the Canon EF-S 55-250mm f/4.0-5.6 lens from someone, however, they told me that they purchased it more than two years ago. I am concerned that some of the new productions of this same model may have been upgraded since the original models. Is this a valid concern or are the models pretty much the same regardless of how old they are?
There do appear to be some production runs that are better than others. As a really general rule, it seems like the earliest product runs have a somewhat greater chance of of having problems.
If it's really only 2 years old, you're probably pretty safe from this being really early production when you have the highest likelihood of running into problems. OTOH, if it's really more like 2 1/2 years old, or was (for example) bought at a retail store where it may have been in stock for six months, you're starting to get a lot closer to one of the early production runs where problems are more likely (though certainly not a given, by any means).
Realistically, damage since purchase is probably more likely than a lens that started out defective anyway. If possible, I'd try to test it out at least a little before buying (but that goes almost regardless of age...)