Hypothesis: the knowledge of making telescopes may be older than "the invention of the telescope", but it was a closely guarded secret and rarely - if ever - applied.
One example: the Welsh mathematician Robert Recorde in his 1551 book on geometry describes how Roger Bacon in the 13thC had "a glass that he made in Oxford, in which men might see things that were done in other places, and that was judged to be done by power of evil spirits. But I know the reason of it to be good and natural, and to be wrought by geometry (since perspective is a part of it) and to stand as well with reason as to see your face in common glass."
Recorde refuses to openly print how it works however, since "this conclusion and other diverse of like sort are more meet for princes, for sundry causes, than for other men, and ought not to be taught commonly".