year-old / year-long
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year-old
Used almost exclusively together with a number to indicate the age of a person or thing in years.
Für die Teilnahme an dem Kurs muss man mindestens 18-jährig sein.
To participate in the course, you must be at least 18 years old.
The adjective 'jährig' is almost never used on its own. It always combines with a number to form a new adjective indicating age or duration, for example 'zehnjährig' or '10-jährig' (ten-year-old / ten-year).
Sein zehnjähriger Sohn spielt gerne Fußball.
His ten-year-old son likes playing soccer.
'Jährig' comes from the noun 'das Jahr' (the year). The ending '-ig' is a typical German ending used to turn a noun into an adjective.
Nach einer zweijährigen Weltreise kam sie wieder nach Hause.
After a two-year trip around the world, she came back home.
Whether '-jährig' describes an age or a duration can be recognized from the noun. If it refers to a person or living being, it is about age. If it refers to a process, a contract, or a thing, it is about duration.
Meine dreijährige Tochter geht jetzt in den Kindergarten.
My three-year-old daughter now goes to kindergarten.
When the number is written as a digit, you use a hyphen: 'der 5-jährige Junge' (the 5-year-old boy). When the number is written as a word, everything is written together: 'der fünfjährige Junge' (the five-year-old boy).
Das 100-jährige Jubiläum der Stadt wird groß gefeiert.
The city's 100-year anniversary is being celebrated in a big way.
long
'lang' and 'jährig' can both have to do with time. 'lang' generally describes that something takes a lot of time: 'eine lange Reise' (a long trip), 'ein langer Kurs' (a long course), 'zehn Jahre lang' (for ten years). 'jährig' is used almost only with a number and usually directly before a noun: 'ein zehnjähriges Kind' (a ten-year-old child) or 'eine zweijährige Ausbildung' (a two-year training program). Learners may confuse the words because both occur in statements involving years. The important point is: for duration, people often say 'zwei Jahre lang' (for two years). As an adjective before a noun with a number, you say 'zweijährig' or 'zwei Jahre alt', especially for age.
Meine Schwester besucht mit ihrem fünfjährigen Sohn die Großeltern.
My sister is visiting the grandparents with her five-year-old son.
Der Sprachkurs dauert drei Monate lang.
The language course lasts three months.