Writer's Fallout Shelter

Don't dump all the details into the opening scene, and don't tease the reader by hinting at details you've no intention of revealing - ever.

Specificity and detail do make characters come alive. There's no need to describe every freckle on a face or every crack in the ceiling, though. There's no need to describe the entire town and name every inhabitant of it before the action can begin.

Read a published novel you like, and one you dislike, with the idea of determining just how much detail is enough.

I know it's a cruel thing to answer your question with "just enough and not too much" but that's the honest answer. Is it important to know the character's age? Can you show us without spelling it out? Try this:

Describe for me an encounter between a 13 year old girl, a 65 year old woman, and a 30 year old man. Don't tell me how old any of them are. Show me - you can use description, dialogue, action - but don't spell it out. Can you work it in so it's interesting? Does it really matter how old they are? (It might, if it's a romance - it might not, if it's a teacher and student. We only need to know one's probably older, more educated, and in a position of authority.)