I've attached a workbook that shows one way of doing this. It includes commentary which I've included here as well. Table calculations can be a bit tricky, especially if you are relatively new to Tableau, so feel free to ask if you have any questions. I'd be happy to answer.
Here are the steps I took:
1. Create a calculated field using the Index() function that is computed using all the fields in the view and ordered along Purchase Date (right click the Index field on the Rows shelf and select "Edit Table Calculation" to see the setup).
2. Create a calculated field based on the Size() function to give us the size of the window.
3. Create another calculated field that divides Index by Window Size to give us the percent.
4. Create a parameter to allow the end-user to control the percent.
5. Create a new calculated field to compare the value of [Oldest %] to the parameter value and use this as the filter.
Oldest n percent.twbx.zip 54.9 KB
Thanks Joshua! This helped a lot!
What I ended up doing (before I saw your response), was exporting the dataset to a CSV, sorting by the purchase date, taking the number of rows divided by 5, making a new column called "Quintile", and then putting a 1, 2, 3, 4, or 5 in the column as needed. Fortunately there were only 3000 or so rows, so it only took a couple minutes to do.
While it isn't as flexible as what you showed me, it got the job done. I still learned a lot from your response! Cheers!