In "Eric is four years old," would you consider "four years old" to be an adjective because it is describing Eric.What I mean is are ages...

This is a complicated question because of the analysis of
"four" and the distinction between (1) class of word or phrase and (2) function of word
or phrase. In the phrase "four years old,"
four is a noun, as dictionaries define
it to be. Yet it functions as an adjective modifying
years and answering how many years:
four.

The phrase "four years old" is
therefore analyzed as an adjective phrase as it begins with an adjective headword:
four. In another approach it might be
tempting to analyze the same phrase as a noun phrase with
four functioning in the capacity of
its other definition as a determiner
(actually, determiners at one time were in the word class of adjectives); with
years in the position of noun phrase
headword; and old finishing the
collocation "years old" as a postpositive adjective of the noun
years.

The
interesting thing is that whether you analyze "four years old" as an adjective phrase or
as a noun phrase, the sentence analysis reveals that "four years old" fills the Subject
Complement slot and adds additional information to the Subject
Eric. Subject Complements may be
filled by either (interestingly enough) an adjective phrase or a noun phrase. The result
is that whether "four years old" is analyzed as an AP or an NP, the same phrase function
of Complement is disclosed.

Adjective
Phrase:
Eric (N; Subject) is (V;
be- linking Verb) {[AP: four
(Adjective Headword) years (N; Complement of Subject) old (Adj; Postpositive Modifier of
Complement)]; Subject Complement}.

Noun
Phrase:
Eric (N; Subject) is (V;
be- linking Verb) {[NP: four (N;
Determiner) years (N; Complement of Subject) old (Adj; Postpositive Modifier of
Complement)]; Subject Complement}.

However, it is not correct to
simply analyze "four years old," in “Eric is four years old,” as an adjective modifying
"Eric." Since the words follow the Verb, then they must be described according to their
function in relation to "Eric": Subject  Complement. Also, simply calling them "an
adjective" confuses the phrase with another function when the words are slightly
modified and in prepositive placement as "four-year-old Eric." In this phrase,
"four-year-old" is a hyphenated compound word that functions as a single unit in the
word class of adjective: "Up at bat was four-year-old Eric." In this usage, the three
words have the same analysis as previously: a noun serving as adjective headword, noun,
postpositive adjective, but since they are a single compound unit positioned before
“Eric,” they function now as a single adjective modifying the noun
"Eric."