The only way that I know of to specify the 60th subject would be to issue 59 0 values and then a 1 for the 60th subject. I am having trouble trying to figure out why you would want to that, but that is for another day.
In order to implement something like selecting the 60th subject, I would use a macro to generate the subject specification coefficients. Accordingly, you could code:
%macro SubSpecEst(subject=);
%let length_subj=%sysfunc(length(&subject));
%let lastdigit=%sysfunc(substr(&subject,&length_subj));
%if &lastdigit=1 %then %let superscript=st; %else
%if &lastdigit=2 %then %let superscript=nd; %else
%if &lastdigit=3 %then %let superscript=rd; %else
%let superscript=th;
estimate "&subject.&superscript. subject slope" time 1 | time 1 / subject
%do i=1 %to %eval(&subject.-1); 0%end; 1;
%mend SubSpecEst;
(Note that I changed your intercept specification in the estimate statement to a time specification because the label indicated the slope for the 60th subject.)
With this macro, one would then write:
proc mixed;
class subjects;
model y=time;
random intercept time/ sub=subjects type=un;
%SubSpecEst(subject=60)
run;