Get off the lawn!
At this point in my life if you are younger, act younger, look younger, are inexperienced at something I might very well refer to you as a kid, the kid, a pup, rookie, newbie etc. If I'm considered a "Get Off The Lawn!" old fart so be it. In more than one setting social or work as the youngest or newest in/to the group I frequently got "Ah, you're just a pup, just a kid" etc and never thought twice about it.
Behavior in combination with age in this case and others merit ' the kid ' or 'these kids now a days'.