Why is this reflexive verb conjugated in passé composé ?

Question

The quiz question about translating "When I was young, people dressed differently" is answered as: "Quand j'étais jeune, les gens s'habillaient différemment."

But I thought the reflexive participle only changes to reflect quantity and gender as in these examples:

Il s'est levé.
Je me suis amusée. (When the speaker is female)
Elle s'est lavée.

Why is "se habille" conjugated into "s'habillaient" in the quiz question above? Shouldn't it be:

"Quand j'étais jeune, les gens se sont habillés différemment"?

Accepted Answer

Hi Sagar,
You can only use the imperfect ( imparfait) here as it describes something people did (used to do) in the past.
It is not a question of agreement, as the s'habillaient is in the third person plural (they) it is just the tense that is wrong.
Look at the lesson below for further understanding:
https://french.kwiziq.com/revision/grammar/l-imparfait-usage-expressing-habits-or-repeated-actions-in-the-past-imperfect-tense
Hope this helps!

Answer

Hi Sagar,
s'habillaient is the imperfect, se sont habillés would be passé composé. 
-- Chris.

Answer

Thank you, Cécile and Chris.
I understand now. It really comes down to the idea that l'imparfait should be used when describing an ongoing action in the past. I was under the impression that l'imparfait was only required for the first part of the answer ("j'etais"), while the passé composé could be used for the second part.It's clear now. Thank you so much.
I love Lawless French.