Noble Coming Out
The Coming Out Event
Nobles under the age of 17 tend not to appear at large noble social engagements, unless that engagement is thrown by their family. The reason for this is simple: they have not yet had official Coming Out events yet. Coming Out events are the formal means by which Waterdhavian nobility introduce their newly adult scions to the rest of Waterdhavian society. They are always grand, festive affairs, with the degree of opulence matching the scion's position in the House, and that House's position in Waterdhavian society. They are the go-to social events, particularly for younger folk, and with good reason - everyone wants to meet the new player on the board, whether to simply make a new friend, scout them out for romantic potential, judge their fitness for use in intrigues and schemes, or even to evaluate for potential future marriage.
There is, of course, nothing stopping an eager young Waterdhavian noble from showing up to social events before their official Coming Out, save for a simple truism - "The face already known is not eagerly sought after". That is, there's no practical reason to attend the Coming Out of a young noble who everyone has already met informally while they've snuck out to events they oughtn't be at. A poorly-attended Coming Out is a disastrous introduction to society that some people work their entire lives to recover from, socially speaking, and the nobility of Waterdeep are vicious. If they had to wait for their formal Coming Out, anyone else who does not do so will undoubtedly feel their scorn.
Coming Out events are always scheduled for after the Presentation Ball, held in Piergeiron's Palace on Mirtul the 4th. To host a Coming Out before then is simply not done, unless the family in question intends a grievous social insult to the Open Lord of Waterdeep (though, in all fairness, the few times this has happened, Piergeiron seems not to have noticed at all). After that date, though, is a flurry of parties and events. The closer an event is held to the Presentation Ball, the more confidence in their importance the family in question is expressing, for their party is bound to be in competition with others, and it is a tragic and ill-omened thing to have a Coming Out event poorly attended.