The Dark Ages is really just a product of transition, as well as being a product of comming after the 90s which I consider the Golden age of animation.
Basically a lot of great shows ended their run at around the 2000s, a lot at once, without a lot of great content to replace it. Nuking entire genres of cartoons for years. Add in immitations of those shows without the heart that ran them and you pretty much see why things got so bad there.
As well there started to be an increased demand of life action over animation. So many of the companies who made cartoons got into making live action. (Not to mention an EXTREME oversaturation of comedy)
There is probably some other things involved but simply speaking it was basically a "Animation depression"
Most people consider the "dark days" of children's cartoons to be anything made after they got too old for children's cartoons
I consider it to be sort of post 2000s where the general quality of cartoons dropped like a rock. a 10 year period of almost nothing.
We are starting the outswing now, but I am not banking on us being "out" just yet.
In 20 years people will be looking back and wondering why cartoons aren't as good as Gravity Falls, MLP:FiM, Adventure Time, etc
Good thing I am fair and only attribute the Dark Ages as the period after 2000 until 2010 and don't consider the 80s inferior in anyway shape or form.