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.
If any difference is noteworthy, it would probably be that an age is usually between two times, e.g. 1290-1450AD, whereas an era is less set - but that isn't a rule, its just a matter of usage. Synonyms would be epoch, or eon. Source(s):
Like epoch, in English it originally meant the starting point of an age; meaning system of chronological notation is c.1640s; that of historical period is from 1741, e.g. the U.S. Era of Good Feeling (which was anything but) in reference to the Monroe Administration (1817-24), attested from 1817.