We youngsters admired our grandmother very much.
You ignorant don't understand what I am talking about.
Can I use an adjective as appositive, as in the second sentence?
No you can't use an adjective in this fashion. You need two noun(s/ phrases), so that the second noun can further define what you meant by the first. You need to turn ignorant into a noun like the ignorant.
"Youngsters" is a noun. As for "ignorant," it would only work if you were to render it as "the ignorant," an ellipsis of the noun phrase "the ignorant ones" or "the ignorant people." (Also note that, as a nonessential appositive, "the ignorant" would be offset with commas.) Words derive their type, or part of speech, from the context in which they are expressed. We may label a particular usage as "correct" or "incorrect" based on our understanding of current norms, but that's often a moving target as language changes over time.
*You ignorant don't understand what I am talking about.
This isn't possible, you have to nominalize the adjective somehow:
You ignoramuses don't understand what I am talking about.
You ignorant people don't understand what I am talking about.
"Ignorant" could just about function as a noun (note the question mark here means these are "questionable") if you make it plural:
?You ignorants don't understand what I am talking about.
This usage could be extended:
?What a bunch of ignorants!
Sometimes nouns are created from adjectives, like "reds" as in "reds under the bed" or "greens", but e.g. "hot" somehow doesn't make it as an adjective, so we have "hotties" rather than
What makes you think these are adjectives? In the first example of yours, you even use the plural marker for nouns. So maybe these are, in fact, nouns. So now, the question becomes
It seems to be not a simple matter, since semantics play an important role here. The following are all examples of what doesn't work:
*You fine should not go in there
*You expensive should work somewhere else
*You funny do not care whether I'm sad or not