From my experience, people who are under 13-14, despite their time of experience, tend to have less ability than those that are older. Like Fifkee said, this could be from maturity, or just straight up more development of the brain. This is not to say that it's not always the case, but, in general and within my area of experience helping scripters, this is how it appears to be.
Also, two years means nothing in terms of how good you are at programming. I could say I've been programming for ten years and it mean nothing (though it would probably imply more than two years does). The reason is that your skill depends not only on brain development (as I've already stated), but also on your dedication to learning. If the expansion of your knowledge happens rarely (ie, you're never challenged to learn new things) and you only stay inside your current bubble of knowledge during the majority of your two years, then your two years means nothing. Thus, it's not good to state how long you've been programming, but to show evidence of your skill. The reason that employers ask for years of experience in the real world is because they expect an answer that is based on how long you've been working at a real company (which would mean your skills would be dedicated 5+ days a week 8 or so hours a day), thus implying that you actually have accrued skill during that work time (unlike the inconsistent improvement that, say, a 12 year old would have had over two years). No offense, but it's hard to be dedicated and focused at that age.
The combination of these and more factors just lends itself to asking for an older age. I can give more reasons, if you ask, but Fifkee has already named one of them, and adding more seems unnecessary. Honestly, if I were hiring, I would go for 16-17 or older. 16 is pushing the lower boundary, to be honest.