Questions & Answers about Kaninen er fem år gammel.
Do I have to include gammel at the end of the sentence?
No, it is completely optional. Just like in English, you can say "The rabbit is five years old" (Kaninen er fem år gammel) or simply "The rabbit is five" (Kaninen er fem år). However, notice that even if you drop gammel, you still must include år (years).
Since the sentence says "five years", shouldn't år be in a plural form?
It actually is plural here! In Danish, many short neuter nouns (words that take et) do not change their spelling when they become plural. So et år is "one year", and fem år is "five years".
Why do we use the singular gammel instead of the plural gamle if we are talking about five years?
Because gammel (old) describes the rabbit, not the years. Since there is only one rabbit (kaninen), the adjective stays in its singular form. If we were talking about two rabbits, we would use the plural form gamle.
How do we know Kaninen means "the rabbit" and not "a rabbit"?
The basic word for "a rabbit" is en kanin. To say "the", Danish takes that article and attaches it to the end of the noun. So, en kanin becomes kaninen.