I've assumed you've data set like this
Using MAKEDATE() I'll convert Emigration Year field to date field.
DOE: MAKEDATE([Emigration Year],1,1)
Now, will take difference of year from both DOB and DOE and then we'll create a bin from that
Year Difference: DATEDIFF('year',DATETRUNC('year',[DOB]),[DOE])
Create bin from the meaures (Year Difference) set the bin size 20
Now drag the created bins in columns shelf and drag person field in rows and take the distinct count
workbook attached for your reference, let us know if this help else share your sample data in packaged workbook.
Book3_v10.2.twbx 9.1 KB