zehnjährig
ten-year-old
Having an age of ten years.
Meine zehnjährige Nichte hat mir ein Bild gemalt.
My ten-year-old niece drew me a picture.

ten-year-old / ten-year
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ten-year-old
Having an age of ten years.
Meine zehnjährige Nichte hat mir ein Bild gemalt.
My ten-year-old niece drew me a picture.

The adjective is made up of the number 'zehn' (ten) and the noun 'Jahr' (year). The ending '-ig' turns it into an adjective meaning 'lasting ten years' or 'ten years old'.
Die zehnjährige Freundschaft der beiden ist etwas Besonderes.
The two people's ten-year friendship is something special.
The noun tells you whether age or duration is meant. If 'zehnjährig' (ten-year-old / ten-year) refers to a person or an animal, it is about age. If it refers to a thing, a process, or a period of time, it is about duration.
Ihr zehnjähriger Sohn geht in die vierte Klasse.
Her ten-year-old son is in fourth grade.
The adjective 'zehnjährig' (ten-year-old / ten-year) usually stands directly before a noun. Using it after a verb like 'sein' (to be) is grammatically correct, but it is less common in everyday speech and can sound formal.
Das ist ein zehnjähriges Auto.
That is a ten-year-old car.
The structure of 'zehnjährig' (ten-year-old / ten-year) is a typical pattern. You can form many similar adjectives with other numbers, such as 'fünfjährig' (five-year-old / five-year), 'zwanzigjährig' (twenty-year-old / twenty-year), or 'einjährig' (one-year-old / one-year).
Das Unternehmen feiert sein fünfzigjähriges Bestehen.
The company is celebrating its fiftieth anniversary.