I am a bit confused about the word "youthism." I was given the meaning "cult of/obsession with youth" by two English-German online dictionaries and Google Books, but wiktionary gives the meaning "discrimination against young people" and lists two quotations. I haven't found the word in the online editions of the renowned monolingual dictionaries. Are both aforementioned meanings still in use or is one of them rather rare?
Youthism is a neologism, constructed from youth + ism.
The 'cult' reading comes from the family of words that includes buddhism, mormonism and confucianism, all of which are related to schools of thought or have religious overtones. People who subscribe to them are very much 'for' (not 'against') the philosophy of their respective root words.
The 'discrimination' reading comes from the family of words that includes sexism and racism. They are labels associated with discussions on discrimination. Those painted with such labels are accused of being unfair on the grounds of the respective root words.
It is arguable that even though the resulting word youthism is spelt the same way regardless of derivation, the words should be considered to be distinct. This is the case for all homographs. Unfortunately, because they are homographs, it's not possible to tell which version was intended by spelling alone. You need the context for disambiguation.