Not everyone who is suitable to take part in a study are able to consent for themselves. Reasons for this might include they are too young, they have a mental health condition or they might be incapacitated in some way, for example, unconscious when a decision needs to be made.
The rules surrounding the ability of children to consent for themselves to take part in a trial are quite grey, and the decision is usually based on the researcher‟s confidence that the child understands what the trial is about and what might or will happen during the trial. Typically however, a researcher will ask a child to agree to a trial (assent), as well as the parents to agree for their child to take part in a trial (consent).
The USA regulatory agency, the FDA, have written an article to explain the reason for including children in clinical trials that might help you make a decision about whether you are interested in having your child take part in a trial.
The approach for consenting adults that may not be able to understand a trial or consent for themselves is similar to that for children. If local regulations allow, a carer/guardian might be able to consent on behalf of the potential volunteer. In emergency situations, where an immediate decision is necessary and no carer or guardian is available, doctors may be able to consent on behalf of a volunteer, although the volunteer would be asked to re-consent when they are able. There will be strict guidelines in place as to who can make these decisions and the process they need to follow.