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?)