He was born with legs bent/curved inward.
By the time he was born, it was apparent that he had bent legs.
But then, it could be a figure of speech here, altough one can be born with bent legs; generally, people "develop" when their diet is lacking vitamin D, phosphorus and calcium along with the habit of people trying to "help" kids/their children to walk earlier than they bone development would allow.
So, some more sentences:
Soon (some time) after he/she was born,
Since tender age he had bowed legs/legs curved inward.
Since early age he had...
When curved outward we also may call them bowed legs.
We call them "crooked/bent" to curved inward or outward legs.