The bottom line is this. If you're 15 years old and hacking any kind of code (yes, Javascript counts), then you're probably in pretty good shape for the future. There's no shame in being a 15-year-old hack, so long as you develop at least a few more tricks before you turn 65. At that point you've missed the boat and death will catch up before you do. So for all the 15-year-old javascript hacks, I present:
How not to become a 50 year old JavaScript hack.
JavaScript is fun. It's a very effective gateway drug for web development. Not quite so much as Flash, but definitely more than CSS. For the type of projects 15 year olds typically do, JavaScript can provide a lot of bang for the buck. Run with it! Learn how to make it work across browsers and platforms, or at least on Mozilla. Implement every half-baked idea that comes into your head. Push the envelope!
Along the way you may find that you have more than a passing interest in the Web. Branch out from JavaScript and slapdash HTML into full W3C-inspired creation. Start with the requisite web technologies like CSS and XHTML, but don't stop there. Become a Photoshop expert, learn how to configure Apache, and study a good server-side language. Get into the world of XML and web services. Build Flash animations if that sort of thing appeals to you.
Once you have a little perspective, the true power of JavaScript becomes apparent. It's not in the gawdy DHTML rollover animation madness of yesteryear, or the ill advised attempts to offload server processing to the client. The power of JavaScript is its singular ability to subtly manipulate a web page without a reloading anything from the server. When implemented correctly it need not detract accessibility or provoke epileptic seizures. JavaScript is for the little things that make a website more usable, like pre-populated secondary menus and basic input validation.
See, JavaScript is only a miniscule part of the Web, but the Web is huge and growing fast. With all due respect to the old-timers, there is enough diversity in the Web to keep even the generalest of generalists satisfied for the rest of their life. The Web shares the timeless combination of simplicity and ubiquity that will ensure its survival in one form or another for a very long time to come.
Technologies come and go, but the Web has become more than a simple protocol and markup language. It's an infrastructure; one that is recognized by most of the general public. unlikely, but the evolution of the Web will be incremental and easily followed. Why? Because the anarchic web with it's open protocols and free programs is too big to be accelerated quickly. The end users will demand a smooth transition. Though the Web is still being pioneered, it is already the defacto standard of multimedia publishing. The significance of that fact is easily lost as we take the Web more and more for granted, but for those with interest in these things, many promising careers are on the rise.
The Web has a finger on every area of traditional computer science along with heavy doses of communication and graphic design mixed in. It bridges the worlds of design-centric QuarkXPress publication, the heavily engineered PC application, and the populous Microsoft Word document. When you combine fields this diverse, you're bound to raise new issues. Usability, interface design, and information architecture are just a few buzzwords that represent huge problem sets in the web development.
Don't come to the web as a designer or a programmer, the results of a singular path are numerous and mediocre. Embrace the new medium for its diversity. Once you look at web development from more than one perspective (eg. technical and visual) you are quickly approaching unexplored territory. Be a pioneer! Be a designer and a programmer! Stick it out and make a name for yourself! The Web is just beginning.