It is true that ex-prisoners can become normal, productive members of society. I completely agree with the idea that allowing such people to speak to teenagers about their experiences is the best way to discourage them from breaking the law.
In my opinion, teenagers are more likely to accept advice from someone who can speak from experience. Reformed offenders can tell young people about how they became involved in crime, the dangers of a criminal lifestyle, and what life in prison is really like. They can also dispel any ideas that teenagers may have about criminals leading glamorous lives. While adolescents are often indifferent to the guidance given by older people, I imagine that most of them would be extremely keen to hear the stories of an ex-offender. The vivid and perhaps shocking nature of these stories is likely to have a powerful impact.
The alternatives to using reformed criminals to educate teenagers about crime would be much less effective. One option would be for police officers to visit schools and talk to young people. This could be useful in terms of informing teens about what happens to lawbreakers when they are caught, but young people are often reluctant to take advice from figures of authority. A second option would be for school teachers to speak to their students about crime, but I doubt that students would see teachers as credible sources of information about this topic. Finally, educational films might be informative, but there would be no opportunity for young people to interact and ask questions.
In conclusion, I fully support the view that people who have turned their lives around after serving a prison sentence could help to deter teenagers from committing crimes.