You would expect that a minor or a party to a proceeding who is declared mentally incapable to manage his/her property and/or personal care (under sections 6 and 45 of the Substitute Decisions Act) would not be able to or required to participate in the litigation. However, this is not so.
Pursuant to Rule 31.03 (5)(b) of the Rules of Civil Procedure, a party under “disability” (defined to include minors and mentally incapable adults) can be examined for discovery if he/she is "competent to give evidence".
The onus of establishing incompetence rests on the party alleging it: Barnes v. Kirk,  2 O.R. 213 (C.A.).
Application of the Rule has led to varying decisions and approaches, a few of which I note below.
· a party under disability may be examined if competent to give evidence subject to the discretion of the court to impose limits where the examination would be oppressive, vexatious or unnecessary: Nyilas v. Janos (1985), 50 C.P.C. 91 (Ont. Master);
· an appointment for discovery should be struck out on the grounds of unsoundness of mind only in the clearest cases – the preferable course is to allow the trial judge to rule on the admissibility of the examination and the credibility of the witness: McGowan v. Haslehurst (1977), 17 O.R. (2d) 440 (H.C.);
· the right to examine a minor for discovery is not absolute – the court should interview the child before exercising its discretion in that regard: Bennett v. Hartemink (1983), 42 C.P.C. 33 (Ont. H.C.);
· a defendant was denied the right to examine a ten-year-old plaintiff where it was found that the examination would result in psychological herm to the child: Kidd v. Lake (1998), 42 O.R. (3d) 312 (Gen. Div.); and
· the court permitted the examination of two plaintiffs (ages 16 and 11) notwithstanding evidence that it might cause serious psychological damage: Nyilas v. Janos, supra.
Have a great day,