Céline,
You should be able to use commas in the CONTRAST statement to create a 2 DF test. But you cannot use them in the ESTIMATE statement.
contrast 'Baby vs Child vs Teenager at T0' group 1 -1 0 group*time 1 0 -1 0 0 0 , group 1 0 -1 group*time 1 0 0 0 -1 0 , group 0 1 -1 group*time 0 0 1 0 -1 0;
See the description of the CONTRAST and ESTIMATE statements in the GENMOD documentation:
9.1:
http://support.sas.com/onlinedoc/913/getDoc/en/statug.hlp/genmod_index.htm
9.2:
http://support.sas.com/documentation/cdl/en/statug/63033/HTML/default/genmod_toc.htm
But if you want multiple comparisons, you would presumably want separate CONTRAST statements for each of the three comparisons so that you get a separate 1 DF test for each pairwise group comparison.
contrast 'Baby vs Child at T0' group 1 -1 0 group*time 1 0 -1 0 0 0;
contrast 'Baby vs Teenager at T0' group 1 0 -1 group*time 1 0 0 0 -1 0;
contrast 'Child vs Teenager at T0' group 0 1 -1 group*time 0 0 1 0 -1 0;
Note that you might find it more convenient to write an equivalent model that has parameters that are more directly interpretable. For example:
model &response = time group(time) / dist=mult;
The 1st parameter of the nested effect compares the 1st and 3rd groups in the 1st time. The 2nd parameter compares the 2nd and 3rd groups in the 1st time. And similarly for the 4th and 5th parameters.