、。
She looks young, but she's actually older than you.
Literal
She [topic-は] young-looking appears [but-が], actually you [than-より] older is.
The が (but) sets up a contrast between appearance and reality. 若そうに見える (looks young) layers ~そう (looks like) onto 若い, then adds に見える (appears to be), so the impression is doubly hedged. 実際は (in reality) flags the correction, and あなたより年上 uses the comparative より — she's above you in age. The polite です at the end softens the statement when addressing the listener directly.