Jacen Solo is born five minutes after his twin sister Jaina on Coruscant. This occurs about five years after Return of the Jedi. The are sent to live at various safe havens, under the protection of Leia's handmaiden, Winter.
In our timeline, the twins are returned to Coruscant at age four and will have many adventures. Sometimes, they will became lost in the underworld of Imperial City, but they are always rescued and returned to their mother.
Jacen has a strong connection to the Living Force — the concentration on the present moment — and can communicate with and calm many jungle creatures. He even keeps a small menagerie of animals in his room. He and his twin sister share. He and his twin sister befriend the warrior princess Tenel Ka.