"The majority read newspapers."
Translation:Majoritatea citește ziare.
Why is majoritatea treated as a sg here when in another sentence (the majority is seventy years old) it took a plural verb ?
Both are fine, but not equivalent. Whether you use a singular or a plural verb with majoritatea is a matter of intention.
If you want to emphasize the group as a unit, use the verb in singular:
Majoritatea a votat împotriva lui. = The majority voted against him.
If you want to emphasize the members of that group, use the verb in plural:
Majoritatea sunt blonzi. = The majority are blond.
The agreement between this noun and verbs is still a matter of debate among Romanian speakers.