Information about youngest child with inner subquery
You should use minimum age in the select clause so that group by is meaningful. Also, why do you need the motherId and fatherId to be null in nested subquery? If you do not add these conditions then you do not need a nested subquery:

select parents.name as name, min(kids.age) as age 
from
people kids
inner JOIN
people  parents
ON (parents.id = kids.motherId OR parents.id = kids.fatherId)
group by parents.id


By the way, does the query you posted really works? In PostgreSQL 9.3.23 I am getting an error:


  column "parents.name" must appear in the GROUP BY clause or be used in
  an aggregate function