siebenjährig
seven-year-old
Describes a person or living being that is seven years old.
Meine siebenjährige Nichte kommt nächstes Jahr in die Schule.
My seven-year-old niece is starting school next year.

seven-year-old / seven-year
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seven-year-old
Describes a person or living being that is seven years old.
Meine siebenjährige Nichte kommt nächstes Jahr in die Schule.
My seven-year-old niece is starting school next year.

Adjectives for age or duration are often formed with a number and the ending '-jährig' (-year-old / -year). This pattern is very productive, for example 'dreijährig' (three-year-old / three-year), 'zehnjährig' (ten-year-old / ten-year), or 'hundertjährig' (hundred-year-old / hundred-year).
Für die zehnjährige Mitgliedschaft erhielt er eine Urkunde.
He received a certificate for his ten-year membership.
You can tell whether age or duration is meant from the noun. If 'siebenjährig' refers to a person or animal, it means age. If it refers to a thing, process, or event, it means duration.
Der siebenjährige Junge kann schon lesen.
The seven-year-old boy can already read.
To state a person's age, the phrase 'ist sieben Jahre alt' (is seven years old) is much more common than 'ist siebenjährig' (is seven-year-old). The adjective 'siebenjährig' is usually used directly before a noun.
Das siebenjährige Mädchen spielt im Garten.
The seven-year-old girl is playing in the garden.