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. 1. the duration, or the measure of time, of the existence of a person or object. 2. the measure of an attribute relative to the chronological age of an average normal individual. achievement age the age of a person expressed as the chronologic age of a normal person showing the same proficiency in study.