"Cannot Group That Selection" Grouping Error

I used to by able to Group on a particular column, but now I can't. I'm grouping on a number column. Using the Column and Row Labels filters on the Pivot Table, I've filtered out any row that would be ungroupable (blanks, anything not a number). I'm sure that the Pivot Table range does not over-extend the data. This worked the other day, but now it doesn't! Do I just have to make a new spreadsheet and remove the ungroupable rows?

Thanks.

Can you show me an example of the table you have set up and I can likely help you with it had the same issue with my pivot table awhile back but without seeing what it is hard to know exactly

Sure: excelforum example.xlsx

I've turned all blanks to '?'. Even though the age columns filter out '?', Grouping on age still doesn't work.

I have three columns (gender, age, rent) and the problem is that I have missing entries in each. Some rows have only gender, some have gender and age, some have all three, etc. How can I group on age or rent without having to create and maintain new spreadsheets that leave out rows that are missing rent and age data?