Who should be baptized?
Anyone that is born again, regenerated of heart, and has invited Jesus to be their personal Lord and Savior. Baptism is not the means of salvation but a response by the Christian of being saved. It is an outward showing of what God has done inwardly. We see examples in the Book of Acts of those that were baptized right after their conversion; like the Ethiopian eunuch and the household of Cornelius (Acts 8:35-36, 10:44-48). They responded to the joy of their salvation and forgiveness by identifying with Jesus, the One who saved them.
When should we be baptized?
Anyone that is old enough to grasp their own need for a Savior and forgiveness of sins is able to get baptized. For younger children, their age of accountability and understanding may be different, so their parents would be the best gauge for them. If they have made a commitment to Jesus as their Savior and understand what it means to be baptized then nothing should withhold them.