You need to use the keyword statement. That lets you apply a style (and other things) to keywords such as all, or even to statistics like mean.
To have the bold apply to the entire column, you can use [style=<parent>] to bring the style of all down, or you can apply it directly inline.
proc tabulate data=sashelp.class;
class sex age;
var height;
keyword all/style={fontweight=bold};
keyword mean;
*Options - first brings the style from the all keyword, second applies directly.;
tables (sex=''),height*(mean='') * (age='' all=Total)*[style=<parent>];
tables (sex=''),height*(mean='') * (age='' all=Total*[style={fontweight=bold}]);
run;