TTest wants to test your mean against a value. Typically that means a Class variable to provide two groups and the test is are the means of the group equal. So if you have a variable that indicates "case" or "control" the syntax would be to test the means between two groups. If you don't have such a variable, then add one to your data set.
proc ttest data= work.query;
class casecontrolvariablename;
var Baseage;
run;
To get a pvalue you must test against something.
You can provide a value to test your mean against with the H0= option on the Proc statement. For example the code below will test if your mean for the Baseage variable is 21.3 (picked at random to provide a syntax example).
proc ttest data= work.query H0= 21.3;
var Baseage;
run;
You should know your data and provide some value that has a use.