Understanding Society User Support

We are analysing the wave 3 question ‘age you think when you leave home’ (lvhm). We noticed that there are a significant number of ‘don't knows’ recorded in the dataset and would like to learn more about them, for example if ‘don't know’ is given more often by certain ethnic groups. Thus we have a couple of questions:

1) Is the answer option ‘don’t know’ explicitly given TO THE RESPONDENT by the interviewer or is this option only recorded when the respondent says this spontaneously?
2) How do interviewers deal with respondents who have no intention to leave home (for whatever reason) – were the interviewers given special instructions?