Responses
Actually it took him 20 years to come back from the dead. It took him 10 years to kinda reform, and 10 years to be able to take a "human shape." See this chapter where he fights the fake version of himself http://www.mangago.me/read-manga/ubel_blatt/mf/v00/c003/ and this chapter which goes over his betrayal and reincarnation http://www.mangago.me/read-manga/ubel_blatt/mf/v01/c001/5/ for more detail.
Ascheritz became blatt master around 12-14 thats why people say he is a prodigy, and they left on the quest to save the world around 20-22 so then died and took 10 yrs to recover after surviving.....so he should be around 30 or so when he is koinzell.(Correct me i'm wrong.)