If you think about it, all languages and all language isolates (there are many others besides Basque, like Ainu) are really the same age. At any rate, this applies if you assume that people developed language only once. Each language then represents an unbroken chain of transmission to the language of the earliest humans to develop speech, or one branch leading back to the original trunk. Isolates are merely branches that have no surviving twigs of recent provenance – recent enough, that is, to discern genetic relationships with the tools available today.
Language isolates of different ages could only really occur if there were several populations of humans developing speech independently of each other, and at different times.
Language isolates of different ages could only really occur if there were several populations of humans developing speech independently of each other, and at different times.