。
She showed me poems she had written when she was young.
Literal
She [topic-は] young days [at-に] wrote poems [object-を] me [to-に] showed [for me-てくれた].
若いころに書いた詩 is a relative clause: 'poems (that she) wrote when young' — the clause 若いころに書いた modifies 詩 (poems). 見せてくれた uses ~てくれる to mark the showing as a favor done for the speaker's benefit — she kindly showed them. The sentence connects past creative work to a present moment of sharing, with a warm, personal tone.