The gland most active during the early years of life and plays a critical role in developing the immune system is the thymus gland.

Here's why:

* T-cell maturation: The thymus is the primary site where T-cells, a crucial type of white blood cell, mature and learn to distinguish between foreign invaders and the body's own cells.

* Immune system development: The thymus is essential for the development of a healthy and functioning immune system. It is particularly active in childhood, producing a large number of T-cells that are crucial for fighting infections and diseases.

* Involution: While it is most active in childhood, the thymus starts to shrink (involute) during puberty and adulthood. However, it continues to play a role in immune function throughout life.

Let me know if you have any other questions about the thymus or the immune system!