YOU SAID:

my mum is 29 when i was 8

INTO JAPANESE

私の母は私が8歳のときに29歳です

BACK INTO ENGLISH

My mother is 29 when I was 8

INTO JAPANESE

私が8歳の時、母は29歳です。

BACK INTO ENGLISH

When I was 8 years old, my mother was 29 years old.

INTO JAPANESE

私が8歳の時、母は29歳でした。

BACK INTO ENGLISH

When I was eight, my mother was 29.

INTO JAPANESE

私が8歳の時、母は29歳でした。

BACK INTO ENGLISH

When I was eight, my mother was 29.