What has amazed me most about this thread is the mention of lasting a few (6, 4) years as being a good sign in toilet seats/lids. I'd expect a toilet seat and lid to last at least thrity years before I thought its lasting worth a mention... like my mum's downstairs loo whose seat has lasted all the fifty-one years she has lived in the same house, and it wasn't at all new when we moved in! It might be the original from 1938 when the house was built.
It's made of wood.
The one upstairs, which was 'posher', had to be replaced at some point but it was probably about fifty years old by that time.