。
She's five years younger than me.
Literal
She [topic-は] me [than-より] five-years-of-age younger is.
The mirror image of an age-comparison with 年上 — here the structure is 年下 ('younger,' literally 'year-below'). Combined directly with the number+counter for the gap (五歳年下 'five years younger'), this is the standard frame for stating age differences. The polite ~です raises the register slightly above plain だ. Mixed-script Arabic numeral ５ within an otherwise kanji-and-kana sentence is common in modern Japanese for spans, prices, and measurements.