1. not fully grown or developed
2. deficient in maturity; lacking wisdom, insight, emotional stability, etc.
3. (geography) a less common term for youthful, used as meaning in the early stage of the cycle of erosion, characterized by steep slopes, lack of flood plains, and V-shaped valleys (for a river, valley, or land surface)
Kicked out of his fraternity
For particularly immature pranks
He went off in a huff and studied
The whole curriculum on his own