Yes, MAJORITIES is a Scrabble word!

Letter values in Scrabble US: M 3, A 1, J 8, O 1, R 1, I 1, T 1, I 1, E 1, S 1.

Is it a valid word?

MAJORITIES is a valid word in the official Scrabble dictionary (US), the UK Scrabble Dictionary and Words with Friends (ENABLE).

the property resulting from being or relating to the greater in number of two parts; the main part
(elections) more than half of the votes
the age at which persons are considered competent to manage their own affairs

Also: absolute majority, legal age.