I'm trying to create a custom field named "Age" where it will compute for the difference between the date today and another field which has another date. How do you do this in NS? I'm thinking custom field type: decimal ❓ but I'm stuck at the formula to use.

You can use the field type of integer or decimal. Then use the formula {today} - {other date field}

This doesn't work

You probably need to do a workflow to do this. The field sourcing tab will not work with formulas unless the data for both fields in the formula is present Before Load