Unfortunately, it's not some punk teenager anymore... These are becoming well-organized, strategically planned attacks. Cyberterrorism, if you will.
There are way too many zombied PCs out there on the net being hosted by people running Windows not properly secured. A large scale DoS attack is easily orchestrated with control over many PCs on varied networks.
Their IP address(es) are probably spoofed at this point and it's impossible to trace these back to the source (ie. in the case of the DoS coming across from an anonymizing proxy like Tor).
Unfortunately, until we start running a more secure Internet, things like this will persist.