I will be 78 years old this year and no longer have the strength to use a compound bow. I can use a rifle unassited. With my doctors certificate can I get a cross bow licence for that special season?

You can get a handicapped archery permit that will allow you to use a crossbow during the archery-only season.  You need to use the IDFG form to apply for it; your doctor can sign it.  Call 208-334-3700 (Fish and Game Headquarters); they will mail the application form to you.