Sarah Davey, now 18, and Lisa Healey, now aged 19, were convicted of the murder of Lily Lilley, 71, from Failsworth, Greater Manchester, three years ago.
England's most senior judge, Lord Chief Justice Woolf, named them at the Court of Appeal in London, as he set the minimum amount of time they must stay in prison.
Both the girls will serve at least eight years for the killing, committed in September 1998.
Squirted shampoo
Each blamed the other for the murder of Mrs Lilley, a widow, at her home.
The pensioner choked on her own dentures when she was gagged by the girls.
They knew Mrs Lilley and after she met the pair on the street, they walked her home.
They squirted shampoo in her eyes and bundled her into the house, before gagging her.
Drugs test
The pair later put the woman's body in a wheelie bin and pushed it into the Rochdale Canal, where it was discovered three days later.
Lord Woolf said he had considered both prison reports and legal arguments on behalf of the girls.
He said Healey had failed a drugs test in custody, but was viewed by prison staff as quiet and polite, and had apologised for what she had done.
Lord Woolf added that Davey's behaviour in custody was described as "mixed", and that she only had a "limited understanding or acceptance of the gravity of the offence".
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