An adjective appears to be used as a noun when denoting an animate plural and preceded by the definite article:
'The successful are those who strive.'
'The foolish are those who procrastinate.'
Is this always the case?
Yep. Because there is always an assumed "people" attached. So long as it's an adjective that can reasonably be used to describe people, you should never run into an issue.
If you ever feel uncomfortable with your construction, just substitute "X people" for your "the X" and see if your sentence still works grammatically.
No, you cannot always assume that. For instance, adjectives ending with -y or -ish usually cannot be pluralized and used as nouns. The adjective 'watery' cannot be turned into 'wateries'. These exceptions are probably because the adjective is already created from a noun and most of these cannot be further altered and turned back into a noun.
In 'The successful are those who strive' successful is not being used as a noun. It is a nominal adjective as in 'the meek shall inherit the earth'. Its meaning is 'all those who are successful'. Another way to view this is that the noun has been elided, as in 'I like the hopeless cat, he likes the sucessful'.
That being the case, if 'the [X]' means 'all that are [x]', it has to behave as a plural count noun or a mass noun.