Is it permissible to rely on a vision seen by one's grandfather in his youth and to prepare oneself for what came in it, and is it permissible to perform Istikhara to pursue that path at the age of 16?

A vision does not establish anything, nor can it be relied upon, for its interpretation might be contrary to its apparent meaning. A righteous person may also see jumbled dreams. The visions of prophets are all truthful, and the visions of the righteous are mostly truthful. As for others, such as the obscure, the wicked, and the disbelievers, truthfulness is scarce in their visions, and jumbled dreams are frequent. If what the grandfather saw was commendable, there is no objection to performing Istikhara and striving to achieve it, for a believer is keen on good and on worldly and otherworldly benefits.