There are 3 ways to get the last day of the previous month. The first is to set the Print Date to be the end of the previous month, rather than 'Today'. The second is to create a date Parameter and have the person printing the report enter the date manually. The last is to use the LastDayOfMonth formula calculate this for you:
LastDayOfMonth (Date(Year(Today), Month(Today)-1, Day(Today)))

Once you have determined the last day of the previous month, you need to calculate the aging periods 0,30,60,90 manually, rather than using the built-in formulas. This is because you want to base the aging on a day other than today. Here are examples for how the 31-60 formula works; the others will be similar:

1) Print Date: If Document date in [(PrintDate - 60) to (PrintDate - 31)] then Current Amount else 0

2) Parameter: If Document date in [(?DateParameter - 60) to (?DateParameter - 31) then Current Amount else 0

3) LastDayOfMonth: The same as the 2 above, but replace 'PrintDate' with the formula in bold.