I have a dataset with 4 time points: 3months, 6months, 1year, 2years and two conditions within each timepoint: WT and KO. I wish to find diff expressed genes between WT and KO from 3months to 6 months, 6 months to 1year, 1year to 2years, 3months to 2years, 3months to 1year, 6months to 2years. I used the design formula below:

> coldata$Age_condition <- factor(paste0(coldata$Age, "-", coldata$condition))

> dds <-DESeqDataSetFromMatrix(countData = countdata, colData = coldata, design = ~ Age_condition)

> dds

And got the following results:

[1] "Intercept"           "Age_condition1YR.KO"
[3] "Age_condition1YR.WT" "Age_condition2YR.KO"
[5] "Age_condition2YR.WT" "Age_condition3M.KO" 
[7] "Age_condition3M.WT"  "Age_condition6M.KO" 
[9] "Age_condition6M.WT" 

How do I contrast to get the genes I want? This is where I am stuck. Thank you.