believer's Baptism

Some honest questions for those of you who practice "Believer's Baptism" Theology. 
1. When can you trust that a person genuinely believes in Christ as Lord and Savior? Do you prohibit younger children, say 5 years old, from being baptized because they may not "fully understand"? 
2. What about mental competence or person's with illnesses that impair cognitive reasoning and understanding? For example, what about an individual who has severe mental retardation? Or an individual who suffers brain damage and has not yet been saved? 
3. What if a person decides to stop believing? Such as a person who was. Altized as a teenager or young adult and later, after going to college or being away from home for a long time, stops living as a Christian or converts to another religion?