Why do people get "too old to learn"?
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).