I was just putting together my A/R report when it occured to me that it would be really handy to have receivables grouped by the age (30 days from now, 60 days, 90 days, 180 days, 1 year, etc.)

The only way I can think of is to have a calculation field on the receivable like this:

Case(Status(CurrentDate) - receivableDate < 30, 1, Status(CurrentDate) - receivableDate < 60, 2, Status(CurrentDate) - receivableDate < 90, 3... etc.

Then group and subsummarize by that field in the report.

Is that the best way? Would this be slow if there were a lot of records because Status(CurrentDate) would have to recalculate for each record?

Also, would I be correct in assuming this should be an unstored calculation, so that the age will recalculate every day?

You're basically correct

The alternative to an unstored fields is to run a script at startup re-calculating the field ... this might be useful if you know you're never interested in invoices older than say 2-3 months so that you might restrict the number of records to be processed