Kalasarpa Yoga is formed by the situation of all planets inbetween Rahu ( Dragons head ) and Ketu in the horoscope ( Dragons Tail ). This leads to  general weakness of the birth chart with the good effects of favourable planets curtailed while the bad effects of evil planets are magnified. 
The combination works on a lifelong basis however the majority of the effects are felt by 36 years of age and thereafter the person progresses well in life.