Since every six consecutive friends adds to the same amount, it follow that 1-6 totals the same as 2-7, and since these two groups have five people in common it follows that 1 is the same age as 7. By the same reasoning, every person is the same age as the person 6 seats away. So 1, 7, 13 and 19 are all the same age. But the group (19,20,21,1,2,3) has the same total as (20,21,1,2,3,4), so 19 and 4 are the same age. Following this through the ages repeat in a pattern every three people, and each consecutive group of three people must add up to 100.

We are given the ages of 1 and 8 and asked for the age of 15. But 8 is the same age as 2, and 15 is the same age as 3, and together they will form a consecutive group of three adding to 100. If the first two are aged 25 and 33, the third must be aged 42.

The person seated at position 15 is aged 42.