。
She's five years younger than him.
Literal
She [topic-は] him [than-より] five years age-below is.
Specifying the age difference: just place the duration (５才) directly before 年下 ('younger'). No special preposition needed. ５才 uses the counter 才 (or its equivalent 歳, both read さい), which counts years of age. Arabic numerals are typical in modern Japanese for specific quantities. 才 is the simplified, everyday writing of 歳; both are pronounced さい and used interchangeably, though 歳 is more formal/standard in writing.