What part of speech is the word "another"?

Determiners are noun markers and clarify nouns. "Another" belongs to the category of general determiners with "a", "an" , "any" etc., the plural of which is " other". "Another" is actually an indefinite article in combination with "other"(an+) and written as a single word meant to suggest an indefinite set/group or just 'a'/'an' as indefinite as they are, but with a twist. Articles, possessives, numericals, quantifiers are all adjectives with a difference. Actually determiners are used to mean proximity, relationship, quantity and definiteness. Quantifiers tell us how many.

In the given example "five" is a quantifier. Another precedes 'five'. When "another" is placed before ' five' , its power of limiting extends over both 'five' and 'year' but serves the same function of an indefinite article which surprisingly A/An cannot fulfil. Wikipedia is right as it notes that 'another' serves the determining function but is more likely to be classified as an adjective in that it generally takes another determiner to complete the phrase although it still comes before other adjectives.

So ANOTHER is a general determiner with a difference.

I think another is a determiner in "another five years". We can also say "the five years" and the obviously is a determiner. Five evidently doesn't have to occupy the same "slot" before the noun as another. You can explain this various ways. BillJ's comment seems to say that "five" is not playing the role of a determiner here, but the role of "modifier". Or maybe there is more than one possible determiner slot before the noun.

There are many different kinds of "determiner-words"/"determinatives", such as articles, demonstratives, and quantifiers, and some of them can be used together.