zweijährig
two-year-old
Having an age of two years.
Unser Sohn ist jetzt zweijährig und kommt bald in die Kita.
Our son is now two years old and will soon start daycare.

two-year-old / two-year
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two-year-old
Having an age of two years.
Unser Sohn ist jetzt zweijährig und kommt bald in die Kita.
Our son is now two years old and will soon start daycare.

The adjective is formed from the number ('zwei' — two) and the noun ('Jahr' — year) with the ending '-ig.' This structure is very productive and works with many other numbers, e.g. 'dreijährig' (three-year-old/three-year), 'zehnjährig' (ten-year-old/ten-year), or 'einjährig' (one-year-old/one-year).
Das Projekt ist auf eine fünfjährige Laufzeit ausgelegt.
The project is designed for a five-year duration.
The meaning depends on the context. If the adjective refers to a living being (person, animal), it usually means 'two years old.' If it refers to a process, contract, or thing, it means 'lasting two years.'
Mein zweijähriger Hund ist sehr verspielt.
My two-year-old dog is very playful.
In the meaning 'two years old,' the adjective is very often used attributively, that is, before a noun. It then describes a characteristic of the person or animal.
In der Spielgruppe sind nur zweijährige Kinder.
There are only two-year-old children in the playgroup.
When it describes a duration, the adjective can stand both before a noun (attributively) and after 'sein' (to be, predicatively). It describes the length of a period of time.
Nach einer zweijährigen Pause kehrte sie in ihren Beruf zurück.
After a two-year break, she returned to her profession.
one-year-old / one-year
'einjährig' and 'zweijährig' look very similar, but they do not mean the same thing. 'einjährig' refers to an age or duration of one year. 'zweijährig' refers to an age of two years or a duration of two years. The words are especially easy to confuse because only the first part is different. So pay attention to the number: 'ein-' stands for one, and 'zwei-' stands for two.

Für das Projekt ist eine zweijährige Ausbildung geplant.
A two-year training program is planned for the project.
Das einjährige Kind läuft schon ein paar Schritte allein.
The one-year-old child can already walk a few steps alone.