Earliest and latest onset of Huntington's Disease(Chorea)?
Huntington's disease may become symptomatic starting from as low as 2 years old [1] and rarely after 55 years old [2] (yet there is evidence of onset at 65 years old [3] and even more according to newer studies).



Figure: Copyright © 2004, The American Society for Experimental NeuroTherapeutics, Inc.
Richard H. Myers. Huntington’s Disease Genetics. NeuroRx. Apr 2004; 1(2): 255–262. doi:  10.1602/neurorx.1.2.255. PMCID: PMC534940

Factors [4]:


CAG number (65%-71% of the variance in age of onset)
siblingship (11%-19%)



  [...] paternal age is negatively associated with age at onset of HD (Huntington's disease), particularly among subjects who inherit the mutant gene from grandfathers [5].




References:


Huntington's Disease Society Of America. What is Huntington's Disease (HD)? Available from http://www.hdsa.org/about/our-mission/what-is-hd.html (accessed 26.10.2014)
National Institute of Neurological Disorders and Stroke. Huntington's Disease: Hope Through Research Available from http://www.ninds.nih.gov/disorders/huntington/detail_huntington.htm (accessed 26.10.2014)
Myers RH, Madden JJ, Teague JL, Falek A. Factors related to onset age of Huntington disease. Am J Hum Genet. 1982 May;34(3):481-8. PMID: 6211092. Full text available from http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1685335/
Rosenblatt A, Brinkman RR, Liang KY, Almqvist EW, Margolis RL, Huang CY, Sherr M, Franz ML, Abbott MH, Hayden MR, Ross CA. Familial influence on age of onset among siblings with Huntington disease. Am J Med Genet. 2001 Jul 8;105(5):399-403. PMID: 11449389. Available from http://www.ncbi.nlm.nih.gov/pubmed/11449389
Farrer LA, Cupples LA, Kiely DK, Conneally PM, Myers RH. Inverse relationship between age at onset of Huntington disease and paternal age suggests involvement of genetic imprinting. Am J Hum Genet. 1992 Mar;50(3):528-35. PMID: 1531729. Full text available from http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1684271/