How to calculate the p-trend of age-adjusted incidences

[lijiaqi]

I need to calculate the p-trend of age-adjusted incidences. The age-adjusted incidences were calculated using the direct method, so the numerators and denominators were large. I tried the poison model and found the p-trend was <0.0001, but the incidence rates seem to be non-linear, rather than an increased or decreased trend.  Also, I found the value/df are very high (>3500), which should be close to 1 for better accuracy. Is it right to use poison model? Which model is good for calculating the 

 p-trend of age-adjusted incidences?

data a;

input periods cases pyears;

loga=log(pyears);

datalines;

1 10000 50000000

2 25000 50000000

3 20000 50000000

4 15000 50000000

;

run;

proc genmod data=a;

model cases=periods / dist=poisson link=log offset=loga;

run;