My dad

Winter Soldier's brain was severely damaged by electroshock and died tragically. Dr. Stark (4997 universe) extracted his soul, and a biologist Tony from the 1030 universe recreated his body and nurtured him back to life.

However, the Winter Soldier's trauma deeply penetrated his soul, making the reborn Bucky fragile, weak, and possibly sickly (though Steve didn't give him much chance to be ill). Steve never let him go out.

Until one day they went to a mountain vacation.

Bucky started being sexually abused by his father when he was fifteen years old. Later, the 1030 Tony installed a particle-based body attribute that allowed him to switch between infant, child, teenager, and young adult stages. He hoped for freedom but couldn't escape his father.

Steve told him he didn't need social interactions or school, just homeschooling. His life's mission was to eat, sleep, and play in this villa until he turned 80, after which Steve would commit suicide to follow him.