der Dreißiger
thirty-something
A person whose age is between 30 and 39.
Als Dreißiger steht man oft mitten im Berufs- und Familienleben.
As a thirty-something, you are often right in the middle of your career and family life.

thirty-something / thirties
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thirty-something
A person whose age is between 30 and 39.
Als Dreißiger steht man oft mitten im Berufs- und Familienleben.
As a thirty-something, you are often right in the middle of your career and family life.

The word is derived from the number 'dreißig' (thirty). The ending '-er' turns the number into a noun that describes a person or a time period.
Dieses Muster findet man auch bei anderen Jahrzehnten, zum Beispiel „die Zwanziger“ oder „die Vierziger“.
You can also find this pattern with other decades, for example 'die Zwanziger' (the twenties) or 'die Vierziger' (the forties).
To distinguish the meanings, pay attention to the article and the number form. 'Ein Dreißiger' (a thirty-something man) is a man aged 30-39, while 'die Dreißiger' (the thirties, plural) usually refers to the decade.
Mein neuer Kollege ist ein Dreißiger.
My new colleague is a thirty-something.
When talking about a person, you can describe the age more precisely with 'Anfang' (early), 'Mitte' (mid), or 'Ende' (late). These words then come before the number word.
Sie ist Ende Dreißig und sehr erfolgreich in ihrem Beruf.
She is in her late thirties and very successful in her career.
To talk about the decade, the plural form is almost always used with the preposition 'in'. This construction requires the dative case.
In den Dreißigern gab es große wirtschaftliche Probleme.
In the thirties, there were major economic problems.