Make two plots nonparametrically relating age to all of the cumulative proportions or their logits. You can use commands such as the following

(to use the R Hmisc package).

for(i in 1:4)

plsmo (age , sfdm ≥ i, add=i>1, ylim=c(.2 ,.8), ylab = ' Proportion Y ≥ j ' )

for(i in 1:4)

plsmo (age , sfdm ≥ i, add=i>1, fun=qlogis , ylim=qlogis (c(.2 ,.8)), ylab= ' logit ' )

Comment on the linearity of the age effect (which of the two plots do you use?) and on the proportional odds assumption for age, by assessing parallelism in the second plot.