Many regulatory mechanisms are very plastic and shaped at critical windows, the most important of which is at early life (from uterus till 4-6 years, but it depends on what). The baby or the young samples the environment and pathways are consequently regulated (for example the responsivity (sp?) of HPA axis). Then, there are also hormetic effects, as the up-regualtion of defenses against oxydative stress. For sure these are interesting times for research in this area. For sure in a few years Chinese will start some experiments ($1.6 billions invested in genome studies).