A very personal decision! It depends on what you are looking for. I have read of many that where all excited because their 9 month old was fascinated by this or that (varies) and how they are happy they didn't miss that reaction. I am, and have been from an early age, much more of a curmudgeon. I would argue that with all the lights and movement in a place like WDW there is somewhat of an overload of stimuli, however, you can, and do, get the same reaction from a 9 month old by waving a flashlight in front of them. So if you go because of the joy it gives your infant, that's OK, but, the reality is that you just enjoy seeing the reaction to stimuli, so you do it for you, not for the child.
My philosophy is that when they are old enough to walk with grown ups, ask questions, make statements and, to me, more importantly remember parts of what they saw... then it is worth it. For me the magic number was 6 years old. I used that number for my own children and then, years later, with my grandchildren. The number was 6. Once everyone had reached that minimum age we made a real party of it.
I realize that some families consist of more years between children. You cannot hold back on a 12 year old because of a 4 year old sibling. But, as a target point I think that 6 is the golden number.
I also realize that a huge portion of my philosophy is self-centered. Small children are a lot of work. The require a completely different approach to the theme park experience. I didn't go to those places for my kids entirely, I went to them for our family and my personal experience. Small children require a lot of attention by necessity, they don't allow many opportunities for spouses participating together as one has to stay behind while the other goes to an attraction and then trade off. That automatically cuts the ability to experience different things in half. So you honestly determine what experience you are looking for in regards to your personal enjoyment and decide from there. There is no set in concrete way for anyone to follow.