Your question is interesting and important but also pretty general, and I'll take a stab at it.
There is MENSA for gifted adults. Maybe MENSA members on the forum could share their experiences with it.
The "rationalist" community centered around the
Slate Star Codex blog may interest you.
Intellectually gifted adults will form online and in-person communities specific to certain domains. For example, tech has Hacker News, Slashdot, and other forums.
There are online courses at places like EdX and Coursera that one can use to initiate or develop intellectual interests.
You used to have be on the hardcopy preprint list of leading researchers to stay up-to-date with research in a field. Now you can get working papers at places like Arxiv and SSRN.
Universities have public events and extension courses. Libraries and recreation departments offer some intellectually stimulating classes.
