Why do people get "too old to learn"? This may be a stereotype, but many of us encounter a lot of people who insist that they can't learn a new technology because they're too old. Some of these people can be brilliant learners in their youth - doctors, lawyers, prominent businessmen, academics. It's so common that one benchmark of good web design is whether the 'average grandma' can use it.

A part of this might be from the formation of habits - someone who's used to reading the newspaper for 50 years would have trouble replacing that with online news. Another possibility is that they may feel that they have limited years to live and don't want to spend their limited time learning a new skill, or that they would have a poor ROI on these skills.

But do they actually have more difficulty learning from a physical perspective or is it just psychological?
Firstly, the matter of lifestyle is probably a significant factor.  Someone who is an alcoholic their whole life and never tries or learns new things is going to have a different outcome that somebody who is still learning new things and exercising and eating healthy.

That being said, lifestyles equal, there's at least two factors in age-associated reduction in memory and learning.  Firstly, just like hips and shoulders start falling apart, so does the brain (characterized by morphological abnormalities and synapse loss).  Organisms are very complex machines and lots of the equipment is gradually failing throughout our life.  Dementia is a typical result of age-induced wear and tear on the brain.

The other is reduced plasticity.  We start off with a very plastic brain, which allows us to learn our environment.  However, part of the process of learning is to stabilize our learning circuits so that they can't be changed again (i.e. you want memories to stay as robust as possible).  Our whole brain goes through series of this stabilization.  At 4-5 years, pruning removes "unused circuitry", greatly reducing plasticity.  In the early 20's, our frontal lobes finalize and myelinate (and this is thought by some to pertain to why people start practicing better judgment at this point).