Why is it not appropriate to use the adult EQ-5D-3L to calculate the EQ-5D-Y index value?

Until recently, no value sets were available for EQ-5D-Y because research showed that the approach used to value EQ-5D-3L health states for use in adult populations was not appropriate when valuing EQ-5D-Y health states for use in economic evaluations of healthcare interventions for paediatric-age populations. Evidence has showed that health states are valued differently when used to describe an adult or a child, that valuation techniques suitable for valuing ‘adult’ health states may be unsuitable for valuing ‘youth’ health states, and that health state values are affected by the wording used in the instrument (EQ-5D-Y vs EQ-5D-3L). Consequently, EQ-5D-3L value sets should not be used to assign values to EQ-5D-Y health states.