Can I ask questions about the "non-canon" Star Wars Universe on this forum?
For Example: "How old was Jacen Solo when he died?"
That question has nothing to do with the movies and Disney has declared it "non-canon".
And how would I format such questions to be clear what I am asking?
Questions about Legends material from the Star Wars universe are on-topic. (Hell, even questions about fan fiction are on-topic here, and material that used to be considered some level or other of canon is a lot higher up the pecking order than that!)
And how would I format such questions to be clear what I am asking?
You can make clear in the text of your question that you're asking about Legends material as opposed to Disney canon, perhaps with a link or citation to the particular work you're referring to or asking about. We also have a star-wars-legends tag, whose tag wiki info says:
Use this tag if your question references Legends sources and/or to indicate that answers are permitted to use Legends sources.
See also How should we handle Star Wars Q&As post-Disney-canon-revamp? and How should we handle Legends-specific Star Wars Q&As post-Disney-canon-revamp? here on meta.
