An individual can offer an affidavit, as long as they have the mental capacity to understand the seriousness of the oath. The contents of an affidavit reflect the personal knowledge of the individual making the statement. This means that an individual making an affidavit cannot be penalised for failing to include information of which they were not aware.
Personal knowledge can in some circumstances, include personal opinion rather than fact.
In certain cases, an affidavit can be offered on behalf of somebody else. This may be the case in relation to the guardianship of an individual who is severely mentally ill.