From what I know of the English pipe makers around the beginning of the 20th Century, the briar used could have been 250 years old at minimum.
It's a popular romanticism that Barling used centuries old wood. It's not actually true. Not everything improves with age, and that includes Briar. After 50-60 years the capillaries shrink and its suitability for heat dispersion is severely impeded. Rainer Barbi wrote about this in another forum that discussed the idea of ancient briar being better than the stuff currently being harvested.
What Barling went after was a specific grade of Algerian briar with an age range between 80 and 120 years, according to their published literature. They believed that this specific variety was at its best within that age range. Then they air cured it for years, believing that any other method used to speed the seasoning process was inherently inferior.
The only other company with a similar approach to their briar, of which I am aware, was Kaywoodie. Their literature from the 1920's states that Kaywoodie pipes were made from century old air cured briar.