Question

so how about this?
Situation :
A : have you ever seen my post on facebook? It's naive haha. I posted it when I was 6.
B : ____________

what should b say if B want to say that he or she hasn't started using facebook yet when a posted that status.

Clive has given you an example. You try it, and we'll comment on your attempt,

Should B say:
I hadn't started using Facebook so I didn't see your post. Here's my attempt.
but what if b just want to say he or she hasn't started using facebook in a specific time like his or her age 0f 6

Jonathan MacI hadn't started using Facebook so I didn't see your post.

That's fine, as is: I hadn't started using Facebook when I was 6.

thanks
but teacher taught me past perfect tense is used to describe first event to second event or the reason and result .
why we can still use it with " when I was 6"
it's not like a sequence or reason and result issue

We are looking back on a past period extending up to a time when the child was six.