Moreover, this form of genetic aging might explain why adults who grow up in poor or otherwise stressful environments are more likely to develop serious health problems, such as cancer, later in life. Scientists think that part of this phenomenon can be explained by protective caps found on the end of our chromosomes, called telomeres, which grow shorter as we age. According to a new study published in Proceedings of the National Academy of Sciences, children who grow up in disadvantaged environments also experience significant levels of telomere shortening — and this effect is apparent by the time they turn nine years old. They also found that children whose mothers changed partners during their childhood had telomeres that were 40 percent shorter than the children whose family structure remained largely unchanged throughout the first nine years of life.