dreijährig
three-year-old
Being three years old.
Unser dreijähriger Sohn geht seit Kurzem in den Kindergarten.
Our three-year-old son recently started kindergarten.

three-year-old / three-year
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three-year-old
Being three years old.
Unser dreijähriger Sohn geht seit Kurzem in den Kindergarten.
Our three-year-old son recently started kindergarten.

The adjective is made up of the number 'drei' (three) and the suffix '-jährig' (from 'das Jahr' — the year). This structure is very productive and can be used with any number to describe age or duration (e.g. 'zweijährig' — two-year-old/two-year, 'zehnjährig' — ten-year-old/ten-year).
Für diese Stelle ist eine fünfjährige Berufserfahrung erforderlich.
Five years of professional experience are required for this position.
The context makes the difference clear. If the adjective refers to a person or animal, it means age. If it refers to a process, training program, or contract, it refers to duration.
Unser dreijähriger Hund ist sehr verspielt.
Our three-year-old dog is very playful.
'Dreijährig' (three-year-old/three-year) is used like any other adjective before a noun and must be declined accordingly. It offers a compact way to express age or duration.
Die Förderung gilt für alle dreijährigen Kinder.
The funding applies to all three-year-old children.