I believe that Zim is somewhere around 70-80 years old.

According to a brief line (from The Trial episode) by young Zim, he’s about ten when he causes Horrible Painful Overload Day (Part 2).  Looking at his younger design versus his scientist uniform, I can assume enough time has passed for him to age, but probably not that far ahead in time… perhaps 5-10 years?  From there, he moves on to Devastis and receives Invader training and rises in military rank as an Elite.  Whenever Tak was up for testing, which seems to be a scheduled planet-wide event, is when Zim “blew the power block for [the] entire half of the planet”.  Later on, when Tak meets with him on Earth, she specifically states that it’s been fifty years since that fateful day.

So… at the time of HPOD2, he’s 10… he ages to about 15 or 20 by the time he’s in Vort… and he’s between 20 to 30 whenever he begins training at Devastis.  And at around 70 or 80, he goes off on his fake mission to conquer Earth, where Tak eventually meets with him.

This is all just an educated unnecessarily complicated guess.  But, uhh… anyone else think the same way?