What is large and important during youth produces hormones that help to program the immune system?

The answer is The thymus.

The thymus is a primary lymphoid organ that is located in the mediastinum, the space between the lungs. It is responsible for the maturation of T cells, a type of white blood cell that plays a critical role in the body's immune system.

The thymus is largest during youth and gradually decreases in size as people age. This is because the thymus is most active in producing T cells during childhood and adolescence, when the immune system is developing. After puberty, the thymus continues to produce T cells, but at a lower rate.

The hormones produced by the thymus, called thymic hormones, help to program the immune system and ensure that T cells are able to distinguish between the body's own tissues and foreign invaders. These hormones also help to regulate the production and activity of other immune cells.