I would start by copying one of the reports that already shows Open tickets because then you are 95% there. Then you can edit that report and create a calculated field that takes the OpenDate of the ticket and subtracts it from a Current Date variable and add that calculated field onto the report. If you don’t know how to create a calculated field support could probably show you if you need assistance.
Just a little more on this topic. In working with another report, we discovered some filter issues with certain date fields with the word AND in the field name. It causes the SQL query to malfunction and makes it appear that the filter is being ignored. This is being worked on and will be fixed in the next release of Track-It! which will either be 2019 R4 or 2020 R1 depending on the exact release timing with the end of the year so close. In the meantime we used filters on Status = Open instead of looking for an Open Date. I hope this helps.