Yeah, I think it's just companies being on the safe side and distancing themselves from any activity that could in any way be remotely construed as selling sex toys to minors. You know how some of the rest of the population at large is--most people don't know what a packer is, and some of those people are prone to over-reaction.
Though I've also noticed that a lot of online sites in general won't allow minors to order stuff--anything, even not sex-related / penis-related.
Anyway, going back to the original question, while it may be questionable whether it's illegal for a company to sell someone a packer (also depends on the country buyer and seller are from, probably), I certainly don't think it would be illegal anywhere (well, most places that I know of) for a minor to own a packer--like, if he makes his own. And even if he buys it from someone, I don't think the actual state of owning it itself is a problem.