How to determine using Facebook Pixel that the user has remained in the application after 28 days?

Question

contextual advertising

I read the answers and generally understand how the Facebook pixel works. Set it up for the application - everything works.
There is an important question, how to understand and determine in Facebook analytics that after 28 days we still have a user from advertising in the application for 25 years or, for example, 30 years.

Answer 1

To clarify, I’ll ask the question a little differently, how to find out how many users of a particular age category are left on the application after 28 days?