I don't see why at 81 you would be too old to have a biopsy in due course following scans, provided there are no medical contraindication for this. (I have just had a template biopsy at 84 as a precursor to a possible 2nd HIFU procedure).
As well as determining the type of cancer, as was mentioned above, the purpose of a biopsy is to sample suspicious areas and grade and plot any found to be cancerous. Some men, even those quite elderly, prefer to have a treatment that may mean they can postpone or avoid completely having HT or other systemic treatment,