Error(s):

source_file.c: In function ‘getoldest’:
source_file.c:18:24: error: ‘N’ undeclared (first use in this function)
          for(int i=0;i<N;i++){
                        ^
source_file.c:18:24: note: each undeclared identifier is reported only once for each function it appears in
source_file.c:19:24: error: subscripted value is neither array nor pointer nor vector
              if(persons[i].age>Max)
                        ^
source_file.c:19:32: error: ‘Max’ undeclared (first use in this function)
              if(persons[i].age>Max)
                                ^
source_file.c:22:35: error: subscripted value is neither array nor pointer nor vector
          printf("%d  %s\n",persons[Maxage].age,persons[Maxage].person_name);
                                   ^
source_file.c:22:55: error: subscripted value is neither array nor pointer nor vector
          printf("%d  %s\n",persons[Maxage].age,persons[Maxage].person_name);
                                                       ^
source_file.c:17:14: warning: variable ‘Maxage’ set but not used [-Wunused-but-set-variable]
          int Maxage=0; 
              ^
source_file.c: In function ‘main’:
source_file.c:36:15: error: incompatible type for argument 1 of ‘getoldest’
     getoldest(persons->age);
               ^
source_file.c:16:5: note: expected ‘person {aka struct <anonymous>}’ but argument is of type ‘int’
 int getoldest(person persons){
     ^
source_file.c:30:5: warning: ignoring return value of ‘scanf’, declared with attribute warn_unused_result [-Wunused-result]
     scanf("%d",&N);
     ^