Why does it take longer for elderly people to learn new things?

As you get older your brain actually shrinks in size. Different parts of your brain (particularly the hippocampus which is the memory center in your brain) shrink significantly in size which reduces their effectiveness. There is also a reduced production of neurotransmitters (which are the networked communication links within your brain). The myelin sheaths (which work as protecting sheaths to your neurotransmitters) also get weaker which causes signals in the brain to become slower/mixed up. Lastly the cells in your body have divided so many times they become damaged or exhausted which decreases their ability to work effectively.

also if you have certain forms of dementia plaques and tangles block the transmission of messages in the brain via neurotransmitters.