Was discussing this same subject in another forum, concerning a guy who bought a used Arai. Used is the first warning flag, but also it was manufactured in 1997. Taking into account the unknown history and the age, I would not recommend that helmet. But, I do agree with you that it is NOT a hard and fast rule, and you will have to decide for yourself. When I first started riding, I used my sister's full face helmet that was made in probably 1987 or 1988. This was in 2004-2005 or so. So it was at least a 17 year old helmet. The interior materials were starting to crack and flake off and you'd be left with flecks of vinyl and plastic in your hair after removing the helmet. I bought a new one quickly. Petroleum products (read, plastics, vinyls) also degrade when exposed to UV rays, depending on the type of plastic. Sweat and salts from the body definitely aren't good for the materials, either.
So my rule is, I will evaluate my own helmets and decide for myself when I think it needs replacing. I would not hesitate to wear a new-old-stock helmet that had been sitting on the shelf since 2003, if it was up to the safety and comfort standards that I'm looking for. A used helmet, I would be vary weary of in general since I don't know the history of it. The EPS liner is good for one impact with the pavement (which is not the same as a helmet drop), and people are not trustworthy. It's really a case by case basis.
Do as thou wilt.
Joseph "Joey" Stalin
zzr 600, r1100rt-p, klr-650...