der Neunziger, die Neunzigerin
this is a person who is 90 years old. If it is a male person you use the male form »der Neunziger«, if female, then it is »die Neunzigerin«
Nom: Der Neunziger feiert seinen Geburtstag. Die Neunzigerin feiert ihren Geburtstag.
Gen: Dort steht die Torte des Neunzigers. Dort steht die Torte der Neunzigerin.
Dat: Gebt dem Neunziger seine Geschenke! Gebt der Neunzigerin ihre Geschenke!
Akk: Wir haben den Neunziger gesehen. Wir haben die Neunzigerin gesehen.
As you might have noticed, all this examples was singular. When it comes to Plural, you have:
die Neunziger
This can be:
- more than one person being 90 years old.
- the last decade within a century (in most cases 1990-1999, but might also be 1790-1799 or 590-599 depending on the context)
(Be aware, that for the usage as decade you have to define, that the first year of a century is that one who's last digits are »00«. Then a decade is a set of years, who's digits other than the last digit are all equal.)
Persons
Nom: Die Neunziger feiern ihre Geburtstage.
Gen: Dort stehen die Torten der Neunziger.
Dat: Gebt den Neunzigern ihre Geschenke!
Akk: Wir haben die Neunziger gesehen.
Decade
Nom: Die Neunziger waren eine wilde Zeit.
Gen: Laura und Jan sind Kinder der Neunziger.
Dat: Schenkt den Neunzigern mehr Aufmerksamkeit!
Akk: Wir haben die Neunziger erlebt.
Note, that »die Neunziger« is plural (without defined gender), while »der Neunziger« is singluar and male, and »die Neunzigerin« is singular and female.
Never mix up plural number and female gender!
die neunziger Jahre
Here »neunziger« is an adjective, describing a property of »Jahre« (years). With »die neunziger Jahre« you man the same decade of years as with »die Neunziger«. In fact, »die Neunziger« is derived from »die neunziger Jahre« by the same mechanism as in those examples:
Mein großer Sohn -> mein Großer
die neue Kollegin -> die Neue
der junge Mann/Knabe/Gesell -> der Junge
The Mechanism is called »Substantivierung eines attributiven Adjektivs« (nominalization of attributive adjectives):
An adjective is used »attributiv« (as an attribute) when it stands between an article or pronoun and a noun. In this case the adjective is an attribute of that noun, and it must be declined together with the noun. If you perform a nominalization of that attribute, you leave out the noun. So you only have left the article or pronoun and the former adjective/attribute. But now, without the noun, the former attribute turns into a noun itself. This means: Like all nouns, it has to be written with an uppercase first letter.
In some cases (»der Junge«) this nominalization is so widespread and often used, that it did become an independently noun, that in modern German is no longer thought to be a nominalization of an adjective. This is why you can find the noun »der Junge« in every dictionary.
But in most cases (»die Neunziger, mein Großer, die Neue«), when you use this words, you still have in mind, that you use an adjective in a nouns suit. And this is why you won't find such words in dictionaries as nouns, but as adjectives, which makes a big difference in German, since nouns are always written with an uppercase first letter, while adjectives (and all other words) are written with all lowercase letter (as long as they are not the first word in a sentence, at the beginning of a heading or similar cases).
But pay attention:
Die siebziger Jahre waren wirklich verrückt. Dagegen waren die neunziger geradezu brav.
Here you don't use »neunziger« as a noun. Here it is an adjective. It even is an attributive adjective! But where is the noun that should come after the attribute? It is in the sentence before, it is »Jahre«. You could also write:
Die siebziger Jahre waren wirklich verrückt. Dagegen waren die neunziger Jahre geradezu brav.
This is grammatically correct, but it is not the best style because you are repeating words. In this case you can leave out one of the »Jahre«, but since both attributes (siebziger, neunziger) still refer to an existing noun, they keep being adjectives.
You also could do this:
Die siebziger waren wirklich verrückt. Dagegen waren die neunziger Jahre geradezu brav.
Now the attribute in the first sentence has a remote noun. It is in this example in the following sentence, but that doesn't matter. It is there, so »siebziger« stays an adjective.
But watch out:
Die Siebziger waren wirklich verrückt. Dagegen waren die Neunziger geradezu brav.
No »Jahre« anywhere. So here both adjectives have to be nominalized.