Children's fiction query. . Possibly published 70s/80s. Dystopian novel - for older children - about a highly controlled/guarded city where inhabitants are assigned roles. Nobody can leave because the outside is savage/dangerous. On reaching the age of 70 inhabitants are killed/subjected to euthanasia. One family rebels because they do not want to take their grandfather to his death and they escape. They discover that the 'savage' outsiders are in fact friendly and civilised, and the escaped family sit down to a meal with them...