This tooth is dark because it has been in the water, but I'm afraid there is no way to know exactly how old it is. I would say it's modern rather than fossil. I'll leave the debate on how we draw the lines between modern, sub fossil and fossil to the palaeontologists!
It looks to me like the molar of a cow, which isn't surprising given that butchery waste has been thrown into the Thames for nearly 2000 years now. Bones and teeth of domestic animals are really common on the foreshore.