"She is too young to smoke"
I wonder whether to infinitive here modifies 'too' or not.
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.
Pope of the Dictionary.com Forum