I know this is OT really in this group but reading the IOM regulations on the Corpun site, there would not appear to be any difference in the construction of the birch for differing ages of offender. However boys under fourteen were caned (not bare). The regulations stating that the maximum dimensions of the cane were not to exceed four feet and half an inch in thickness. Very curious as that this is exactly the dimensions of the rod used for the most brutal canings possible in Singapore!
This information comes from the data provided by the British Government to the European Court, so should be accurate. Not surprising then that so little is known about IOM judicial caning of schoolboys.