I'm assuming that means you are going up from younger kids to 16s. A lot still will depend on the level of play. Some general thoughts:
- The are largely going to play more physically and accept more contact--pay attention to how they react as you figure out where the foul bar is
- There can be substantial size differences that can be a challenge with that growing contact
- They will (generally) understand advantage
- Some of them are going through a part of life when they want to challenge authority; you need to talk to them something closer to a peer than with the younger kids while still being the one in charge. But many of them understand the game well enough that you can talk to them intelligently
- Some will wear a caution as a badge of honor; you need to use plastic when required, but also need to be aware of how you read the room, so to speak
- Many will have skills that they can often control, but will try things beyond their skill level
- They aren't past doing stupid things--they may still have that moment when they suddenly think they came up with a brand new idea by trying to kick away the ball as a GK punts it
- Get to hot spots quickly--they are often more likely to posture than to actually want to fight--if you get there quickly and loud with the whistle, they are often looking for an excuse to not fight without losing face by backing down
If you haven't, go watch a game at this level before you ref it to get a feel.
Good luck!