。
She looks young, but she's actually over 40.
Literal
She [topic-は] young appears [but-けど] actually 40 [object-を] is-exceeding [explanatory-んだ].
~けど (but) is the casual contrastive conjunction, setting up the surprise: she looks young, but the truth is different. 40を超えている (is exceeding 40) uses the verb 超える (to exceed/surpass) with ~ている for a resulting state. The んだ ending adds explanatory force — the speaker is revealing a surprising fact. This is a very natural, conversational way to express this kind of age-related surprise.