'Sixteenth year'

[Troels Forchhammer]

A question for the British in the group ...

In HPB-17 'A Sluggish Memory', Dumbledore tells Harry about Tom Riddle,

     In the summer of his sixteenth year, he left the
     orphanage to which he returned annually and set off to
     find his Gaunt relatives.

Now, speaking mathematically this must be the summer when Tom was
fifteen years old (think of the child's first year, which is the one
from birth to the first birthday), but I guess there is a possibility
that British colloquial usage is different.

The matter is important since it makes the difference of whether Tom
killed his Riddle family just before or just after he opened the
Chamber of Secrets and caused the death of Myrtle.

So, would the 'sixteenth year' in normal /British/ usage be the year
when the youngster is fifteen or sixteen?

/Troels