Yes, YOUNGEST is a Scrabble word!

Letter values in Scrabble US: Y 4, O 1, U 1, N 1, G 2, E 1, S 1, T 1.

Is it a valid word?

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

(used of living things especially persons) in an early period of life or development or growth
(of crops) harvested at an early stage of development; before complete maturity
suggestive of youth; vigorous and fresh
being in its early stage
not tried or tested by experience