CTRebbe – recall a case within last month of Daf Yomi, must be Shekalim, when someone not satisfied with a person passing halakhic story from a traveler, went to the original source and asked him.
There is also Rav Yehudah disapproving a visiting youngester, told to be a Haham, when he turned out not to wear a kippah/turban because he is still not married. This sounds like an example of unsolicited advice from a Gadol to a person he does not know. But maybe we can start with Gemora examples when someone asks a Rav permission to get married and see what the discussion leads to?
On the opposite side, I saw an interview of R Gifter in the 60s where he says most marital problems are due to marrying too early.