Teaser 2819: An age-old problem

From The Sunday Times, 2nd October 2016 [link]

Five men of different ages under fifty are celebrating their joint birthday in the pub. Alan’s age is the average of their five ages. When Bob is three times Alan’s age today, the sum of Alan’s and Colin’s ages will equal the sum of their five ages today. Furthermore, Derek will then be three times his age today, and Eric will be one year more than double Bob’s age today.

The landlord checked that the youngest of the five was allowed to buy alcohol.

Who is the youngest, and how old is he?

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