Ages!

Riddle: 'When Gwen is twice as old as Dean, Then I shall be just 17. But Gwendoline was 23 When Dean was twice as old as me.' Thats what Bill said. So tell us then, How old he was when Dean was ten?

Answer: Eight.

Source: https://www.riddles.com/3208

'When Gwen is twice as old as Dean, Then I shall be just 17. But Gwendoline was 23 When Dean was twice as old as me.' Thats what Bill said.
So tell us then, How old he was when Dean was ten? — Ages! by don orr v1.

Riddle: 'When Gwen is twice as old as Dean, Then I shall be just 17. But Gwendoline was 23 When Dean was twice as old as me.' Thats what Bill said.
So tell us then, How old he was when Dean was ten? Answer: Eight. — by don orr v2.