But the rule of thumb is: the older the victim, the younger the subject.

What I usually do is start with age 25 and then, based on the level of sophistication of the crime, along with some other factors, I'll either lower or raise it.

Serial murderers generally surface in their mid- to late 20s. That's why when you get someone like 39-year-old Angel Maturino Resendez -- the suspected railway killer formerly known as Rafael Resendez-Ramirez -- the odds are he didn't start killing when he was 37. Resendez, suspected in the deaths of at least nine people, probably began killing somewhere back in his 20s. The more sadistic types are generally older.

Older criminals come prepared

If the crime has a high degree of sophistication and some planning went into it -- maybe he brought a "rape kit" with him -- you're probably looking at an older offender.

If he's thought up some ruse or con, or brought along bindings or a weapon in order to have more control over the victim and the crime itself, he's showing some sophistication. So you can start adding on some years.

Let's say you've got a rapist who brings along a disguise and afterward strips the bedding and forces his victim to take a shower. This would indicate that he has probably had a run-in with police in the past. Now he's thinking far enough ahead to get rid of the evidence. So you're probably not dealing with a young guy.

Older criminals have learned from their mistakes. They've had time to take a look back. They've been incarcerated and learned by talking to other inmates. In the past, maybe they've assaulted their victims and let them live. Now, they may feel it makes more sense to kill them. Or maybe they'll learn a better method of disposing of a body.

In a recent case in the Midwest, the killer dumped his female victim in a shallow grave and then built a fire on top of it. He did this to throw off any scent a dog might pick up and to circumvent infrared cameras that can detect heat from a decomposing body. This indicates a high degree of criminal sophistication.

Younger offenders more impulsive

If a crime is disorganized, impulsive or sloppy, you would lean toward a younger criminal.

When you're dealing with victims who are senior citizens, you're generally looking for a youthful offender -- particularly in cases of sexual assault. These younger guys know they can control them.

Sometimes you'll find that a young offender has done some menial tasks for the person who later becomes his victim -- he has cut the grass or helped out around the house.

Maybe he'll strike the victim during a struggle, using his fist or an object. He doesn't stab the victim with his own knife -- he uses a kitchen knife. That tends to mean the subject didn't come prepared.

Usually, the primary goal is money. The crime may start out as a burglary, but then the light bulb goes on. The subject becomes an opportunist. A burglary turns into a sexual assault or may escalate to murder.