at the age of puberty

pu·ber·ty  [pyoo-ber-tee]  noun
the period or age at which a person is first capable of sexual reproduction of offspring: in common law, presumed to be 14 years in the male and 12 years in the female.

at the age of puberty
思春期に

at the time of puberty
思春期に

before the age of puberty
思春期前に

We all have to go through the painful period of puberty.
私たちはだれしも思春期につらい一時期を経験しなければならない。