I feel like I have a learning disability? Why would it show up at 55?

Many ways. Failure to retain new learning can occur at any age if any of many causes are present. These include drug effects (either prescribed or "recreational"), sleep deprivationj alcohol abuse, depression, head injury, sleep disorders, congestive heart failure, insufficient blood oxygen and neurodegenerative disorders (such as alzheimer's or frontotemporal dementia).
Not usually. It is unusual for a person of your age to develop a learning disability. Speak to your doctor for evaluation of medical problems that can cause your symptoms.