die Altersstufe
age group
A stage in the life of a person or group defined by a specific age.
Dieses Gesellschaftsspiel ist für die Altersstufe ab 12 Jahren empfohlen.
This board game is recommended for the age group 12 and up.

age group
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age group
A stage in the life of a person or group defined by a specific age.
Dieses Gesellschaftsspiel ist für die Altersstufe ab 12 Jahren empfohlen.
This board game is recommended for the age group 12 and up.

This noun is made up of 'Alter' (age) and 'Stufe' (level/stage), connected by an 's'. So it literally describes a stage or level defined by a specific age.
In jeder Altersstufe haben Kinder unterschiedliche Bedürfnisse.
At every age group, children have different needs.
The word is feminine ('die Altersstufe' — age group) because the last word in the compound is 'die Stufe' (level/stage). This rule helps you determine the gender of many long German words.
Die nächste Altersstufe beginnt mit der Pubertät.
The next age group begins with puberty.
Very often, the age group is given with 'ab' (from/and up) or 'bis' (up to) and a number in order to define an age limit. This pattern is often found with games, films, or books.
Der Film ist für die Altersstufe ab 16 Jahren freigegeben.
The film is approved for the age group 16 and up.
Common formulations are that something 'für eine Altersstufe geeignet ist' (is suitable for an age group) or 'einer Altersstufe entspricht' (corresponds to an age group). The word is often used in educational or developmental-psychology contexts.
Diese Aufgaben sind für seine Altersstufe noch zu schwierig.
These tasks are still too difficult for his age group.
age
'das Alter' and 'die Altersstufe' both have to do with years of life. 'das Alter' usually gives the exact number of years, or more generally refers to the life stage of being an old person. 'die Altersstufe', on the other hand, means a group or stage, for example children from six to eight years old or teenagers in a particular school grade. The words can be confused because both appear in forms, school, or sports contexts. If you are asking for a number, 'Alter' fits. If you are dividing people into groups by their age, 'Altersstufe' fits.

Für jede Altersstufe gibt es im Schwimmkurs eine eigene Gruppe.
There is a separate group for each age group in the swimming course.
Beim Anmelden muss Anna ihr Alter angeben.
When registering, Anna has to state her age.