。
Old age creeps up on us before we know it.
Literal
Old-age [topic-は] notice-not while [to-に] us [to-に] creeps-up.
気づかぬうちに uses the classical negative ~ぬ (= ~ない) for literary effect: 'while not noticing.' 忍び寄る ('to creep up, steal upon') is a vivid compound verb. The sentence has a proverbial, contemplative quality about the imperceptible march of time.