Personal circumstances.
The legal fiction of the reasonable person is an ideal, as nobody is perfect. Everyone has limitations, so the standard requires only that people act similarly to how "a reasonable person under the circumstances" would, as if their limitations were themselves circumstances. As such, courts require that the reasonable person be viewed as having the same limitations as the defendant.
For example, a disabled defendant is held to a standard that, by necessity, represents how a reasonable person with that same disability would act. This is no excuse for poor judgment, or trying to act beyond one's abilities. Were it so, there would be as many standards as there were defendants; and courts would spend innumerable hours, and the parties much more money, on determining that particular defendant's reasonableness, character, and intelligence.
By using the reasonable person standard, courts instead use an objective tool and avoid such subjective evaluations. The result is a standard that allows the law to behave in a uniform, foreseeable, and neutral manner when attempting to determine liability.
Children.
One broad allowance made to the reasonable person standard is for children. The standard here requires that a child act in a similar manner to how a "reasonable person of like age, intelligence, and experience under like circumstances" would act. In many common law systems, children under the age of 6 or 7 are typically exempt from any liability, whether civil or criminal, as they are deemed to be unable to understand the risk involved in their actions. This is called the defense of infancy: in Latin, doli incapax. In some jurisdictions, one of the exceptions to these allowances concern children engaged in what is primarily considered to be high-risk adult activity, such as operating a motor vehicle, and in some jurisdictions, children can also be "tried as an adult" for serious crimes, such as murder, which causes the court to disregard the defendant's age.
Mentally ill.
The reasonable person standard makes no allowance for the mentally ill. Such a refusal goes back to the standard set in Menlove, where Menlove's attorney argued for the subjective standard. In the 170 years since, the law has kept to the legal judgment of having only the single, objective standard. Such judicial adherence sends a message that the mentally ill would do better to refrain from taking risk-creating actions, unless they exercise a heightened degree of self-restraint and precaution, if they intend to avoid liability.
Generally, the courts have reasoned that by not accepting mental illness as a bar to recovery, a potentially liable third party, such as a caregiver, will be more likely to protect the public. The courts have also stated the reason that members of the public are unable to identify a mentally ill person, as they can a child or someone with a physical disability.
