der Siebziger
septuagenarian
A male person whose age is between 70 and 79.
Mein Großvater ist ein rüstiger Siebziger, der noch jeden Tag spazieren geht.
My grandfather is a spry septuagenarian who still goes for a walk every day.

septuagenarian / seventies
Loading your saved state.
septuagenarian
A male person whose age is between 70 and 79.
Mein Großvater ist ein rüstiger Siebziger, der noch jeden Tag spazieren geht.
My grandfather is a spry septuagenarian who still goes for a walk every day.

The word is derived from the number 'siebzig' (seventy). This pattern is used for all decades and age groups, e.g. 'ein Zwanziger' (a person in his twenties) or 'die Achtziger' (the decade 1980-1989).
Die Musik der Achtziger ist immer noch sehr populär.
Eighties music is still very popular.
The article and the number (singular/plural) show the meaning. 'Der Siebziger' in the singular is a male person. 'Die Siebziger' in the plural refers to the decade.
Der nette Herr von nebenan ist schon überzeugter Siebziger.
The nice gentleman next door is already a convinced septuagenarian.
When talking about the decade, the preposition 'in' is often used: 'in den Siebzigern' (in the seventies). In texts, the abbreviation 'in den 70ern' is also often used.
In den Siebzigern gab es viele politische Veränderungen.
In the seventies, there were many political changes.
When it is about the decade, the word is often combined with cultural topics such as 'Musik' (music), 'Mode' (fashion), or 'Filme' (films).
Die Mode der Siebziger war sehr bunt und auffällig.
Seventies fashion was very colorful and eye-catching.
senior
'der Senior' and 'der Siebziger' can both refer to an older male person. 'Senior' is more general and often polite: it usually refers to older people, especially of retirement age, without naming an exact age. 'Siebziger' is more specific and means a man between 70 and 79 years old. The words can be confused because many Siebziger are also senior citizens. But not every Senior is exactly in his seventies. If the age is important, Siebziger fits; if you are speaking generally and politely about older people, Senior is better.

Der Siebziger fährt noch jeden Tag mit dem Fahrrad zum Markt.
The septuagenarian still rides his bike to the market every day.
Im Schwimmbad gibt es morgens einen Kurs für Senioren.
At the swimming pool, there is a class for senior citizens in the morning.