I would guess you can experiment it with the slide button function, in UI screen. Then link different images or actions to either a screen that says you can't play because your underaged and right age to a UI ... I'm not experienced in BuildBox but from what I've seen, I'd start there and see if I can work something out. Hope helps.
Sorry, but this isn't possible! Buildbox is for simple game creation at the moment; you can't store variables. I hope this helps!