I know kids who were OTD then returned in their early twenties. I know kids who are in their thirties now who are still OTD. Every situation is unique and needs to be dealt with sensitively & individually.
Generally, I would advocate welcoming them but asking them to keep their smoking, laptop & phone use in their rooms. As for clothing – again, it depends on the individual. But if a teen knows he is loved regardless and there is a connection with the home & family, there is a much greater chance of return.
One last thing… if a teen from a Chassidishe home decides to cut off his peyos, wear a kippa sruga, but still keep Shabbos & Kashrus, is he OTD? Or his he finding his own derech?