CRC/C/GC/24 34. Only if these measures prove unsuccessful may there be an assessment of the child’s physical and psychological development, conducted by specialist pediatricians or other professionals skilled in evaluating different aspects of development. Such assessments should be carried out in a prompt, child- and gender-sensitive and culturally appropriate manner, including interviews of children and parents or caregivers in a language the child understands. States should refrain from using only medical methods based on, inter alia, bone and dental analysis, which is often inaccurate, due to wide margins of error, and can also be traumatic. The least invasive method of assessment should be applied. In the case of inconclusive evidence, the child or young person is to have the benefit of the doubt. Continuation of child justice measures 35. The Committee recommends that children who turn 18 before completing a diversion programme or non-custodial or custodial measure be permitted to complete the programme, measure or sentence, and not be sent to centres for adults. Offences committed before and after 18 years and offences committed with adults 36. In cases where a young person commits several offences, some occurring before and some after the age of 18 years, States parties should consider providing for procedural rules that allow the child justice system to be applied in respect of all the offences when there are reasonable grounds to do so. 37. In cases where a child commits an offence together with one or more adults, the rules of the child justice system applies to the child, whether they are tried jointly or separately. D. Guarantees for a fair trial 38. Article 40 (2) of the Convention contains an important list of rights and guarantees aimed at ensuring that every child receives fair treatment and trial (see also article 14 of the International Covenant on Civil and Political Rights). It should be noted that these are minimum standards. States parties can and should try to establish and observe higher standards. 39. The Committee emphasizes that continuous and systematic training of professionals in the child justice system is crucial to uphold those guarantees. Such professionals should be able to work in interdisciplinary teams, and should be well informed about the physical, psychological, mental and social development of children and adolescents, as well as about the special needs of the most marginalized children. 40. Safeguards against discrimination are needed from the earliest contact with the criminal justice system and throughout the trial, and discrimination against any group of children requires active redress. In particular, gender-sensitive attention should be paid to girls and to children who are discriminated against on the basis of sexual orientation or gender identity. Accommodation should be made for children with disabilities, which may include physical access to court and other buildings, support for children with psychosocial disabilities, assistance with communication and the reading of documents, and procedural adjustments for testimony. 41. States parties should enact legislation and ensure practices that safeguard children’s rights from the moment of contact with the system, including at the stopping, warning or arrest stage, while in custody of police or other law enforcement agencies, during transfers to and from police stations, places of detention and courts, and during questioning, searches and the taking of evidentiary samples. Records should be kept on the location and condition of the child in all phases and processes. No retroactive application of child justice (art. 40 (2) (a)) 42. No child shall be held guilty of any criminal offence that did not constitute a criminal offence, under national or international law, at the time it was committed. States parties that expand their criminal law provisions to prevent and combat terrorism should ensure that those changes do not result in the retroactive or unintended punishment of children. No child should 8