At which age are ninjas revived with Edo-Tensei? The question is easy: At which age are people, when they are revived using Edo-Tensei? Are they exactly of that age they died? Or can the user of Edo-Tensei decide, how old the ninjas are?
According to the Naruto Wiki page on Edo Tensei, when summoning


  the reincarnated seem to retain any permanent body damage and physical limitations they received during their lifetime.1  


I think this means that normally they would be resurrected with the age they had upon their time of death, as well as physical conditions.  

However, as we know, and the page describes in the enhancements part, (quote follows, as is on the site) 


   Kabuto can also modify his summoned fighters, as he did with Madara Uchiha. While the technique usually reincarnates the deceased at the exact state they were in at the time of their deaths, Kabuto noted that he brought Madara back in a state that was "beyond his prime"2 and was reincarnated much younger than when he died an old man as well as instilling him with abilities he had acquired in his old age.


So, basically, I think that the user may perfect the Jutsu in order to obtain the results he wants, provided that he has enough skill and knowledge on the Jutsu to do it.  


   Or maybe only Kabuto can do it like that.




1 This piece of data is stated in the Wiki, but I do not recall if it was referenced in any manga chapter, so bear that in mind.
2 Naruto chapter 560, page 3.