。
She had her first child at twenty.
Literal
She [topic-は] twenty-years-old's [time-とき] [at-に] first's child [object-を] gave-birth.
二十歳 is read はたち — one of the special readings to know. Twenty is a culturally significant age in Japan (the traditional age of adulthood until 2022; legally 18 since). 産む ('give birth') is for the act of birthing; with kids/babies, you also see 生まれる ('be born') for the baby's perspective.