According to this report, ISIS has declared:
What does the last sentence about young men mean? Is it saying that men can’t be more than 20 years older than their wives?
FYI, in case it’s related, you should know that girls as young as 9 (some claim 6) can marry according to ISIS.
I’m pretty sure that the document refered to in the linked article is one that wasn’t released by ISIS per se, but by an organization of female combatants affiliated to it (and hasn’t been validated by ISIS).
I’ll search if I can find it. But I believe they simply meant that men should marry at a young age too (the document also states that despite 16 or so being an ideal age for girls, they could be married earlier if it’s in their best interest, IIRC).
Here’s a translation of the document [Warning : PDF]. There’s a short analysis of it, and it’s an interesting read about the views held by female jihadists about the role of women, education, etc…
It just means that ideally, girls should marry around 16 (after their education has been completed at 15, except for a few activities, like female doctors) to slightly older young men.
Need answer fast? 
I’m not seeing why you would read that as anything other than “Men should marry before they turn 20 years old”.
By the way, I noticed while searching for it, that Fox News is misrepresenting it (stating that ISIS instructs girls to be married at 9), as if the original content wasn’t bad enough.