Some people might have already known this
The number 7 is said to be luckiest number in the world in many cultures. Well Luffy happens to be 7 years old when he ate gomu-gomu devil fruit and was given straw hat by shanks.
The number 9 is an unlucky number which means to suffer.
Trafalgar law was 9 years old when his family died.
zoro was 9 years old when kuina died.
Alq to some calculations I did nami happens to be 9 or 10 years old when Bell- Mere died.
well, if she was 9 the above fits on her as well.
The next thing could be completely wrong:-----
In Japan no. 49 is also an unlucky number and it means going to die.
Aokiji happens to be 49 years old right now.