Today’s second glitch:
Spell out the numbers instead of using numerals: fourteen, not 14.
Both fourteen-year-old and ninety-year-old are compound adjectives (single adjectives made of more than one word), so they need hyphens.
There’s a comma splice after back together. Change the comma to a period.
A comma is needed after Doing so.
A fourteen-year-old boy wakes up one morning as a ninety-year-old man with amnesia. With no recollection of his life, he slowly begins to piece the memories back together. Doing so, he discovers a dark and terrifying secret from his past.
I’d get rid of With no recollection of his life; saying he has amnesia covers that.