I would think more in time rather than size... Superdwarfs vary in size considerably... I have seen 6 year old females that were not more than 5'6".... I have also seen pure sd females that were 8ft long..... I would think 3-4 years old would be the youngest... My 09 50% sd female (double het albino axanthic) measured her today she is 7'10" (bigger than her momma and siblings)
so I wouldnt think about it in terms of size but age