
das Alter
age
The number of years that a person, animal, or thing has existed.
In welchem Alter hast du Fahrradfahren gelernt?
At what age did you learn to ride a bike?
age
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age
The number of years that a person, animal, or thing has existed.
In welchem Alter hast du Fahrradfahren gelernt?
At what age did you learn to ride a bike?

old age
The final phase of a person's life, the stage of life as an old person.
Viele Menschen wünschen sich, im Alter gesund und aktiv zu bleiben.
Many people want to stay healthy and active in old age.

age / era
A specific historical period or era.
Mit der Erfindung des Internets begann ein neues digitales Alter.
With the invention of the internet, a new digital age began.

Dude!
A colloquial exclamation (form of address) that expresses surprise, annoyance, or excitement. Comparable to 'Man!' or 'Whoa!'.
Alter, hast du das gesehen?
Dude, did you see that?
'Das Alter' (age) is the noun related to the adjective 'alt' (old). It describes the state of being old or the number of years someone has lived.
Mit zunehmendem Alter wird man oft ruhiger.
As people get older, they often become calmer.
'Das Alter' (age) can mean an exact number of years or the stage of life as an old person. If a number is included ('im Alter von 30' — at the age of 30), the number of years is meant. If it stands alone ('im Alter' — in old age), it usually refers to the life stage.
Er wanderte im Alter von 25 Jahren nach Australien aus.
He emigrated to Australia at the age of 25.
To ask a person's age, you almost always use the adjective: 'Wie alt bist du?' (How old are you?). The question 'Was ist dein Alter?' (What is your age?) is uncommon and sounds very unnatural or bureaucratic.
Auf dem Formular müssen Sie Ihren Namen und Ihr Alter eintragen.
On the form, you have to enter your name and your age.
The exclamation 'Alter!' (Dude!) is very informal youth slang. Depending on the tone of voice, it can express surprise, annoyance, or excitement. Use it only among friends.
Alter, was machst du denn hier?
Dude, what are you doing here?
human / person
Mensch! and Alter! can both express surprise, annoyance, or excitement. Alter! is distinctly more youthful and very colloquial. Mensch! is also colloquial, but fits more social situations and is usually less strongly marked.

Alter, das war wirklich knapp!
Dude, that was really close!
Mensch, das war wirklich knapp!
Man, that was really close!