So there you have your three broad categories of education that can help you become a webmaster. None of these is a guaranteed path to success. Clients will always prefer someone with a solid, proven portfolio regardless of education. If you feel you already know what you need to know, build your portfolio, get out there and be a webmaster. But if you feel you're lacking in some areas, find the option that works for you at this point in your life.
For mature individuals who have already spent some years in the workforce, some training courses may be all require. They will give you the specialized skills you need and you can use your work experience to leverage those new skills.
For younger individuals, still in high school or recently completed, I would heavily encourage a two- year program or a four-year degree. If a four-year university degree is a realistic option for you, I would highly encourage anyone to pursue it.
I'll admit that I am biased on this issue, having had a wonderful experience completing my Bachelor of Computer Science at Dalhousie University. I feel it's been instrumental in getting me where I am today. But then again, every webmaster would say the same of their own background, so take any advice on this topic with a grain of salt.
There are many paths to becoming a webmaster. Some have better chances of succeeding, others will provide broader career options if you're not 100% decided on becoming a webmaster. Find a path that works for you and most importanly, enjoy the journey.