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):
A period cannot be a point in time - it is the time elapsed between two points in time. Period does not imply any form of delimiting event or circumstance - it just refers to time itself. 26 days or 11 minutes is a period. Stage and phase both refer to some sort of process. When referring to time, they imply a beginning and an end but also some process outside that period of which the stage or phase forms part.