As Rob said, mostly because it makes them seem more professional.

However, in some countries (United States being one), the title "CEO" is used to describe the highest ranking person within the business/company, regardless of the size. So, it doesn't matter whether you own and run a business single handedly from your bedroom (meaning you are obviously the highest ranking authority within the business), or whether you employ 1000 people to work for you - if you are the highest ranking authority, you are basically the CEO.

In this country (England) I don't believe that is the title is so widely used. Our equivilent is probably a "Managing Director" (correct me if i'm wrong)... and you can't really be a director unless your company is registered (so sole traders don't count), so it doesn't really apply to sole traders (which many of these "14-15" year olds are) I don't believe.

If, on the other hand, these 14-15 year olds reside in a country where it is legal for individuals of this age register a company and/or be appointed as a director, there is no reason why they cannot label themselves as being the CEO (they would have just as much right as everyone else).

But after all that... yeah, most people of this age probably aren't in this position so they are using the term falsly in a hope that it portrays a more professional image.