Comment Re:How's that? (Score 4, Informative) 155
And how do you determine the age of some random rocky mass that you can't even image?
According to the BBC article, they simply guessed the age. The sub-brown dwarf or rogue planet seems to be travelling with a group of stars, and they've estimated the age of the stars to be 50 - 120 million years. It's a form of extra-solar profiling: That thing over there isn't a star, but it's hanging out with those other stars, so it must the same age as them. (Which is apparently OK to do for stars, but not people?)