。
She's about the same age as my older sister.
Literal
She [topic-は] my older-sister [as-と] same-about [possessive-の] age is.
~と同じくらい is the standard equality comparison: 'about the same as.' くらい/ぐらい softens 同じ ('same') from a strict identity claim to an approximate one. Note 姉 here refers to the speaker's own older sister — Japanese kinship terms split by both relative age (姉 vs 妹) and in-group/out-group (姉 for own, お姉さん for someone else's or in direct address). 年齢 is the formal/written word for age; the conversational alternative is 歳 or 年.