[R] survival analysis question

[Phguardiol@aol.com]

Hi,
I m studying the effect of a binary covariate on the time to an event, ie, patient requiring treatment. 
It clearly appears that this covariate has an effect during the first year of life but not anymore thereafter.
I d like to know until what age this covariate has a significant impact on the endpoint of interest, knowing that there is a competing risk here since the patient can also die during the time of study.
What would be the best option to do so ? 
Said in a different way, what I think is that when this covariate is = 1 the risk of requiring a treatment is increased as compared with patients who have this covariate = 0 by 4, 5, 6, 7, 8 years of age but after there is no more impact of this covariate since they will all receive a treatment. The idea is to be able to say, if covariate = 0 then the risk of requiring a treatment until 8 years is lower than if the covariate = 1; and I d like to be able to say until when this risk in significantly increased. Hope I m clear !  
thanks for your help
Philippe