According to Mayo Clinic, Down syndrome causes a distinct facial appearance, intellectual disability, developmental delays, and may be associated with thyroid or heart disease in a person. So calling people with this syndrome 'Mongoloid' is extremely vulgar and disrespectful to ethnic Mongolians. Likewise, how will the population react if Down Syndrome was associated with 'European' looks? If you use this term, you are automatically a xenophobic, in my opinion.
An ERA is a long period of history with distinct characteristics. For example, the roman era. A PERIOD is less distinct and defined, it simply represents a portion of time, be it a very short portion, or a very long portion. For example, my period of sadness and the jurassic period. An EON is an actual unit of time. 1 eon = 1 billion years. This is also used rhetorically to symbolize a long long time. For example, man! my new iphone took like an eon to arrive in the mail! or also more appropriately the earth is 4.5 eons old.
Age is defined as a span of years during which some event occurred. The number of years that ice covered most of the world is an example of an ice age. The definition of age is the number of years something has been alive or in existence. An example of age is being 16 years old. Age means to experience the passage of time or show signs of growing older. 1  To allow wine to ferment and develop its flavor over a certain number of years is an example of the process used to age wine.  An example of age is a person who grows from 15 years old to 60 years old.