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I have read this sentence and got slightly confused.

When my mother, who was only 18 when she had me, told me I should wait until I got older to marry Lucy, I knew she was really happy I met the right person.

— Adam from engvid.com via Wikipedia Republished

In this sentence when my mother is an adverb clause and who was only 18 she had me is an adjective clause.

I know an adverb can modify an adjective, but not vice versa. I believe in this sentence that an adjective clause is modifying an adverb clause.

I am curious to know what this adverb clause is modifying.

When the car
 which was full of circus clowns
passed through the remote village,
the villagers just shook their heads and said "These city folk".

The temporal adjunct clause is "When the car ... passed through the village".

The relative clause "which was full of circus clowns" describes "the car", but the clause is not distinguishing the car from other nearby cars so that you would know which car was meant; the clause is merely a descriptor, extra info.

The main clause is "the villagers just shook their heads and said 'These city folk' "

In English all adverb clauses are introduced by a subordinator "when my mother" is an adverb clause of time(dependent clause). The next grammatical item "who was just 18" is a relative or adjectival clause and it functions as a complement to the Subject "my mother"

Let us look critically at this structure;

1. "When my mother, who (she) was just 18.....,"

Hence, who refers to the antecedent...."my mother" and it functions as the Subject.

So let us look at it in this way
1. ".........she was just 18"
In English only complements (Adjectives) come after a copula or linking verb.
Therefore, making it an an adjective clause.

However, the adjective clause "who was just 18" does not modify the any adverb in this context.

Moreover, the clause when she had me is an adverb clause of time which qualifies the NP my mother.