Especially with the Flea Market option now in City Living, I would like my teen to be able to bring with him the things he bought. Someone shared with me another silly and tedious way to do this, but that works.
After your teen has grown up and moved out, create a little room separate from the house, and put in it whatever you wanted him to take with him. Then save that room to the gallery. When you go to your now grown teenager out on his own, go to the gallery and add this room to his yard. You can then take the items from the room and put them wherever you want in his house, then delete the room. You can likewise delete the room and items from his parents house.
This works with any way you want a Sim to "gift" something to a Sim in a different household, including unique items you cannot buy in build mode.
