Korean is an agglutinative language, which means there are many endings that can be added onto words - especially verbs and adjectives. Therefore, when encountering words and learning them, it's important to recognize the base form, and also learn how to add endings.
In this case, the base of 어린 is 어리, an adjective / descriptive verb meaning "to be young (as a child)"; the dictionary citation form is 어리다. The dictionary form of a verb/adjective will always end in 다, and we can remove the 다 to get the base form, upon which all verb suffixes are attached.
So 어리+ㄴ => 어린 gives us the modifier form of the adjective - that is, the form we can put before a noun:
어린 딸 - young daughter
어린 시절 - the time of (my) youth)
The base form cannot be used by itself, but we can add 아/어 to get the 반말 form - a familiar/informal form that is used with friends or with younger people (avoid using it until you know when it's okay!):
그는 아직 어려 - he/she is still young (어리+어 => 어려)
A more polite version is to add 어요:
그는 아직 어려요
You will normally encounter adjectives and verbs in all sorts of forms, so it's good to learn to identify the base form right away, and also the most common endings - there are dozens of endings, so it takes a while, but once you know the most common endings it will be easy to dissect most forms quickly.