No, el sacamantecas is still scary even when you are 26. It is based on a group of real Spanish serial murders.
The name derives from "sacar" ("to extract") and "manteca" ("body fat"). The crimes that inspired the "sacamantecas" boogeyman were very real and scary. It was a mix of the following criminals:
Romasanta: He lived on the XIXth century, and sold fat and grease for home use (leather preservation, lubricant, etc). Some of his neighbours claimed that the fat he obtained in order to elaborate these products came from muredered humans, so the goverment tried to arrest him. Now, this is when shit got real: he ran away and scaped, killing one policeman and nine people with his own teeth and actually eating parts of their body. He was latter diagnosed with lycantropy (it is an actual mental illness), and his execution sentence was conmuted into life prision in order to study his fascinant mental condition.
Juan Diaz de Garayo: Serial killer that killed up to six prostitutes inflicting horrible wounds to their bellys. Even thought he only killed one children, his fame for his hatred about children had to do with the fact that he was caught by the police due to a small girl: his face was so horrible that she got scared and started screaming her lungs out. People thought that he tried to molest her, and carried him to the nearest police station, where they latter discovered that he was the serial killer.
The "hombre del saco" also existed. It was an actual motherfucker that kidnapped and killed a seven year old children in order to obtain his body fat, which was very priced as it was believed that it acted as a remedy against tuberculosis at the time.
Quite awesome article about the origins of the "hombre del saco" and "sacamantecas" here:
http://tejiendoelmundo.wordpress.com/2009/03/23/mitos-y-leyendas-el-sacamantecas/
