@tinksmama , you are welcome.
Dates in Excel are actually sequential integer numbers starting from 1 which is equivalent of 01 Jan 1900 (for Windows). Thus 43671 that's the sequential number of the date starting from above date. If you apply Date format to that cell it will show 25 July 2019.
You may check if there is no date entered then return invoice age as zero, e.g.
=IF( LEN(A1)=0, 0, TODAY()-A1)
or return empty string "" instead of zero.