1   12 13 14 15 16 17 18
2   21 22 23 24 25 26 27 28 29
... --------

1  15
2  25
....

perl -MList::Util=sum -ane 'printf "%s %sn", shift @F, sum(@F)/(@F?@F:1)' file
1 15
2 25

awk '{ s = 0; for (i = 2; i <= NF; i++) s += $i; print $1, (NF > 1) ? s / (NF - 1) : 0; }' file

1 15
2 25