。
She's six years older than I am.
Literal
She [topic-は] me [than-より] six-years-of-age elder is.
The standard age-comparison frame ~より~歳年上 ('older than X by Y years'), where より sets the basis of comparison and 年上 specifies the senior side. The polite ~です ending raises the register slightly above plain だ, fitting more careful or formal contexts (introducing a sibling to a stranger, talking to a teacher, etc.). The Arabic numeral ６ in mixed-script style is common in modern Japanese for spans, prices, and measurements, even within otherwise kanji-heavy text.