Is a 80 year old person the same person and with the same ideas, abilities, look, as when he was a 15 year old person? He has the same name after all.

Same language doesn't mean you can necessarily speak it without learning the new words, forgetting some stuff that went deprecated, learning some syntactic sugar and new features. If you were born in 1400 and lived to today, you'd be going from speaking from Medieval French to modern French with hardly noticing the difference...