We can use this structure (“the” + adjective) to talk about large groups of people, such as “the sick” meaning “sick people” or “the rich” meaning “rich people.” 

The mayor plans to build more housing for the elderly.

Guided bus tours are especially popular with the elderly.

Note: adding “the” in front of an adjective actually turns it into a noun phrase. For example, “the adventurous” means “(the) adventurous people”.