my @aRecords;
my $iRecord=0;
while ($line = <INPUT_FILE>) {
  my @aFields = split(/,/, $line);
  $aRecords[$iRecord] = \@aFields;  # \@ creates an array reference
  $iRecord++;
}

# now that you have read in all the lines, you can sort them
# assuming age is the second field, it will be stored at
# index 1 (fields are numbered starting at 0)

my @aSortedRecords = sort { $a->[1] <=> $b->[1] } @aRecords;