Twins, triplets, quadruplets would be the words you seek.
people be born at the same time
people of roughly the same age can be called contemporaries.
Synchronised?
The noun is generation, generations is the plural form. Generation is all of the people born and living at about the same time, regarded collectively.
A generation is a group of people born at roughly the same time. A contemporary is someone living in the same time period.
Septuplets
Quintuplets.
generation
it means that different people can do the same thing at the same time
A cohort is a sample of people that are all born at about the same time. For instance, everyone that you are studying are born in 1940. Time measurement effects mean when the time you measure something effects your dependent variable.
When two people yawn at the same time means one is sleepy. The other is just copying the other.
It means at the same time. Such as two people talking simultaneously.