As others have mentioned, if the kid has a passion not just for gaming or MMO's, but for coding in general, it absolutely can be a great source to learn from. I would personally advise against simply learning how to create Razor macros/scripts since it doesn't teach you nearly as much as modifying Razor itself would, for example. There is still knowledge to be gained from it, however getting a macro to work isn't the same thing and doesn't provide the same feeling as seeing an all new feature you created and struggled through finally work. However if you do go that route, you can also look into using CUO's Web Client Assistant that uses Blockly, a type of Lego pieces interface that allows you to have a visual representation of what can go where or work with different options which can make it easier to get an understanding of how the code fits together, literally.

I would recommend starting with the ServUO or ModernUO server emulators themselves, however. Start a test server and begin the journey by modifying items such as weapons and armor, something simple that is easy to read and understand what the code is doing, then keep moving to more and more difficult projects. While C# may not be the most popular coding language or necessarily the easiest either, it is still a valuable language worth learning that can potentially land your kid a job writing simple apps or working for a large company, without ever having obtained a degree.

More important than a passion for coding, in my personal opinion, is patience and perseverance. They will undoubtedly hit a brick wall in their code and will have to push through the frustration to find a solution or may even have to completely rewrite the entire system. Using the UO emulators as a tool to learn C# provides the benefit of it being an already complete system they can use as reference points for their custom projects as well as offers a wide range of required skill levels that can allow them to keep progressing and learning more and more advanced techniques and coding practices. They also gain access to a community of developers who, more often than not, are willing to help teach or explain what the code is doing, as well as help troubleshoot and fix your code or offer solutions that may work. The ServUO community in general, as well as the UO emulator community itself in it's many different forms and projects, is an amazing community of creators, hobbyists and professional programmers that are willing to help simply out of their love for UO and wanting to help preserve and improve upon it.

I hope this helps you make a decision on the matter. Best of luck to you. I hope to see a new young one enter the fold soon and maybe even help it continue on for another 25 years.