Id  ht
510 69
510 67
510 65
510 62
510 59
601 29
601 26
601 21
601 20

Id  Avg.ht
 510 64
 601 24

$ cat FILE
Id  ht
510 69
510 67
510 65
510 62
510 59
601 29
601 26
601 21
601 20

$ awk '
    NR>1{
        arr[$1]   += $2
        count[$1] += 1
    }
    END{
        for (a in arr) {
            print "id avg " a " = " arr[a] / count[a]
        }
    }
' FILE

$ perl -lane '
    END {
        foreach my $key (keys(%hash)) {
            print "id avg $key = " . $hash{$key} / $count{$key};
        }
    }
    if ($. > 1) {
        $hash{$F[0]}  += $F[1];
        $count{$F[0]} += 1;
    }
' FILE

id avg 601 = 24
id avg 510 = 64.4

perl -lane'END{for(keys(%h)){print"$_:".$h{$_}/$c{$_}}}($.>1)&&do{$h{$F[0]}+=$F[1];$c{$F[0]}++}' FILE

#!/usr/bin/perl
use strict;
use warnings;

my %sum_so_far;
my %count_so_far;
while ( <> ) {
    # Skip lines that don't start with a digit
    next if m/^[^d]/;

    # Accumulate the sum and the count
    my @line = split();
    $sum_so_far{$line[0]}   += $line[1];
    $count_so_far{$line[0]} += 1;
}

# Dump the output
print "Id Avg.htn";
foreach my $id ( keys %count_so_far ) {
    my $avg = $sum_so_far{$id}/$count_so_far{$id};
    print " $id $avgn";
}

ire@localhost$ perl make_average.pl input.txt
Id Avg.ht
 510 64.4
 601 24

datamash -H -s -g 1 mean 2 <file

s