I have not been able to find anything to back the hypothesis that there can be a development disorder which will slow body development, but retain mental development rates. However I have seen a slightly different disorder in journals.
Slow body development also with slow mental development
This was reported by Andy Coghlan (2009) which covered the issue with 16 year-old Brooke Greenberg who had a physical development of approx. 5 years old.
Scientists hope that by comparing the genome of a 16-year-old girl locked physically and mentally in a baby-sized body, that they may find the genes for holding onto youth
(see also: Walker, et al. 2009 and Walker, et al. 2015).
We previously reported the unusual case of a teenage girl stricken with multifocal developmental dysfunctions whose physical development was dramatically delayed resulting in her appearing to be a toddler or at best a preschooler, even unto the occasion of her death at the age of 20 years. Her life-long physician felt that the disorder was unique in the world and that future treatments for age-related diseases might emerge from its study (Walker, et al. 2015).
Brooke died aged 20 from Bronchomalacia in her home town of Baltimore, MD. (The Telegraph, 2013)
Coghlan, A. (2009). Teen baby narrows search for “Dorian Gray” gene. New Scientist, 203(2715), 12-12 doi: 10.1016/S0262-4079(09)61749-8
The Telegraph (2013). Brooke Greenberg - obituary, The Telegraph [Online]
Retrieved from: https://www.telegraph.co.uk/news/obituaries/10428714/Brooke-Greenberg-obituary.html
Walker, R. F., Pakula, L. C., Sutcliffe, M. J., Kruk, P. A., Graakjaer, J., & Shay, J. W. (2009). A case study of “disorganized development” and its possible relevance to genetic determinants of aging. Mechanisms of ageing and development, 130(5), 350-356. doi: 10.1016/j.mad.2009.02.003 PMID: 19428454
FREE PDF: http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.565.3771&rep=rep1&type=pdf
Walker, R. F., Liu, J. S., Peters, B. A., Ritz, B. R., Wu, T., Ophoff, R. A., & Horvath, S. (2015). Epigenetic age analysis of children who seem to evade aging. Aging (Albany NY), 7(5), 334–339. doi: 10.18632/aging.100744 PMCID: PMC4468314
Full Accessible Text: https://www.aging-us.com/article/100744/text#fulltext
FREE PDF: https://s3-us-west-1.amazonaws.com/paperchase-aging/pdf/XHhjqD7fdWZMyww4C.pdf