Your problem is that your data structure isn't really right for your problem.
The right data structure is a more vertical structure, with DATE, VAR, VALUE. Then PROC MEANS
is perfect for your needs.
data have;
input date $ var_1 var_2 var_3 var_4 var_5;
datalines;
1957M1 . . . . .
1957M2 . . . . 23.5
1957M3 . 1.2 . . 23.6
1957M4 . 1.3 . . 23.7
1957M5 . 1.4 . 0.123 23.8
1957M6 . 1.5 . 0.124 23.9
1957M7 . 1.6 3.0 0.125 23.10
1957M8 . 1.7 3.1 0.126 23.11
1957M9 . 1.8 3.2 0.127 23.12
1957M10 2.1 1.9 3.3 0.128 23.13
1957M11 2.2 1.10 3.4 0.129 23.14
1957M12 2.3 1.11 3.5 0.130 23.15
;;;;
run;
data want;
set have;
array var_[5];
date_num = mdy(substr(date,6),1,substr(date,1,4));
do _iter= 1 to dim(var_);
if not missing(var_[_iter]) then do;
var = vname(var_[_iter]);
value = var_[_iter];
output;
end;
end;
format date_num MONYY.;
run;
proc means data=want;
class var;
var date_num;
output out=edge_dates min= max= /autoname;
run;