Excluding the royalty factor(Crown Prinz Wilhelm and his brother Prinz Eitel were both Generals at 32),candidates would be:
--born 1870,commissioned in the field 1917.
----born 1869,MajorGeneral Ost 1917.
Fritz Von Lossberg
--born 1868,MajorGeneral 1917.
A scan of the German Army Handbook of WW1
shows that most German divisional commanders were at least over 50 years old during the war.
http://pro.corbis.com/images/watermark/ ... VV1663.jpg
Lettow-Vorbeck,after returning from East Africa,1919.