]ie, only for the past few years. Moreover, Bingley seems to have been of age only for 2 years. Perhaps, he got introduced to Darcy through some common 'connection', and thus their friendship began

Yes I think of that too, and I think because of Mr. B's good,agreeable, and inviting personality Darcy liked him so as a friend. JA shows us in many occasions that Bingley is a considerate and trustworthy fellow, and that what Darcy needs.