Age-related decompensation of pre-existing phoria: This is the most likely cause due to the weakening of extraocular muscles and the reduction in neural control with age, leading to the manifestation of a previously compensated strabismus.

Diabetic third nerve palsy: A common cause of new onset squint in older adults, resulting from microvascular ischemia affecting the third cranial nerve.
Thyroid eye disease: Although more common in younger populations, thyroid disorders can occur at any age and may cause strabismus due to extraocular muscle involvement.
Stroke or cerebral vasculature issues: Vascular events can lead to problems with eye movement and alignment, presenting as a squint.

Giant cell arteritis: A condition that, although rare, is critical to diagnose due to its potential to cause blindness if not treated promptly. It can present with diplopia among other symptoms.
Brain tumor or metastasis: Although less common, a new onset squint could be a presenting symptom of a space-occupying lesion affecting the brainstem or cranial nerves.
Myasthenia gravis: An autoimmune disorder that can cause fluctuating weakness of the extraocular muscles, leading to intermittent squint.

Graves' disease with restrictive myopathy: While thyroid eye disease is more common, the specific presentation with restrictive myopathy causing squint is less frequent.
Ocular myasthenia gravis: A localized form of myasthenia gravis affecting only the eyes, which can cause diplopia and squint.
Intracranial aneurysm: Although rare, an aneurysm pressing on a cranial nerve can cause a squint, making it a diagnosis that should not be overlooked despite its low probability.