maybe something like this:
= count( if( session_date >= account_created_date and session_date <= account_created_date+14, session_date))
to count all session dates within the first 14 days (replace 14 by 30/60 to retrieve the other buckets then).
Create a straight table with dimension customer_id, then use above expression(s). Your sessions should already be correctly linked to the customer via customer_id. session_date and account_created_date must also be recognized as Date type by QlikView, please have a look into date format and Date() and Date#() functions for that.
Does this answer your question? I assumed you already loaded the tables into QlikView and the tables are linked together.