Christian baptism by the immersion in water is (1) your symbolic identification with the death and resurrection of Jesus (Romans 6; Colossians 2:12), and (2) a clear expression of a repentant life to God (1 Peter 3:21). When you are lowered under the water, it signifies identifying with Jesus dying for your sins. Coming up out of the water identifies with his resurrection to newness of life, just as the believer has a new life in Christ (Galations 3:27).
Do we baptize children or infants?
We do not baptize infants because they are not capable of making a conscious decision to accept Jesus Christ as their Savior (1 Peter 3:21). We do not have a set age for children to be baptized; however, we rely on the judgment of parents in deciding when a child is ready. However, we do have dedication ceremonies for infants and children.