Adhesive Capsulitis (Frozen Shoulder): This condition is common in older adults and can cause bilateral shoulder pain and stiffness. The gradual onset of pain over one month also supports this diagnosis.

Osteoarthritis: A common condition in older adults, osteoarthritis can cause bilateral shoulder pain, especially if there is a history of trauma or repetitive strain.
Rotator Cuff Tendinopathy: This condition can cause chronic shoulder pain and is common in older adults, especially if they have a history of heavy lifting or repetitive arm movements.
Polymyalgia Rheumatica: This inflammatory condition can cause bilateral shoulder pain and stiffness, especially in older adults.

Multiple Myeloma: Although rare, multiple myeloma can cause bilateral shoulder pain due to bone lesions. It's essential to consider this diagnosis to avoid missing a potentially life-threatening condition.
Metastatic Cancer: Cancer that has spread to the bones can cause bilateral shoulder pain. A thorough evaluation is necessary to rule out this possibility.
Infectious Arthritis: Septic arthritis or infectious arthritis can cause severe shoulder pain and requires prompt treatment to avoid serious complications.

Amyloidosis: A rare condition that can cause bilateral shoulder pain due to amyloid deposits in the joints.
Hemochromatosis: A genetic disorder that can cause joint pain, including shoulder pain, due to iron accumulation in the joints.
Sjögren's Syndrome: An autoimmune disorder that can cause joint pain, including shoulder pain, due to inflammation and dryness in the joints.