The best available evidence suggests children with CP will reach their maximum gross motor potential before 6 years of age (so by age 5) and children may begin to lose motor skills/function as young as 8 years. This is consistent with the Campbell text. Here is one reference to support these numbers:
Suk MH, Park IK, Yoo S, Kwon JY. The association between motor capacity and motor performance in school-aged children with cerebral palsy: An observational study. J Exerc Sci Fit. 2021 Oct;19(4):223-228. doi: 10.1016/j.jesf.2021.07.002. Epub 2021 Aug 3. PMID: 34447440; PMCID: PMC8361183.