The subject slot in a sentence is typically filled by a nominal – noun phrase, gerund phrase, infinitive phrase, and noun clause. But sometimes adjectives can be subject.
Learn more: Can an adverb or adverbial be subject?
Note that unripened in The unripened apple fell from the tree is not an example of adjective functioning as subject. Here The unripened apple is subject of the sentence.
Adjectives can be the head of a noun phrase (the rich, the elderly), implying it can function not just as subject, but also as object of verb, object of preposition, and complement in a sentence. Adjectives are typically used as head noun to represent established groups of people (rich and elderly represent a group) and are usually preceded by the in such use. Compared to adverbials though, which have limited use as subject and that too informally, adjectives have a far wider use as subject. Examples:
The educated volunteered to help educate the uneducated.
The brave never back down from a challenge.
The Spanish are known for their vibrant festivals.
The Swedish are famous for their innovative designs.
The unforeseen was what led to catastrophe.
