"She is too young to smoke"
I wonder whether to infinitive here modifies 'too' or not.
You asked about what the infinitive "to smoke" modifies in your sentence. Traditionally, every word in a sentence functions as one of the 8 (or 9 or 10 depending on the source) parts of speech. Modifiers are classified based on the words they modify. In that use, "modify" means to change or alter the meaning of the word it modifies. Your sentence contains: pronoun (subject) - verb (linking) - adverb - adjective - to infinitive.
The adverb "too" modifies the adjective "young" and "young" is a predicate adjective modifying the pronoun "she". Without the infinitive, we have "She is too young". That leaves us a question about what she is too young for. The infinitive answers that question. Therefore, "to smoke" modifies "young" and would be classified as an adverb.