Well I think it says a lot about a person if they specifically date younger people. It would mean that probably that person is not as interested in his mate's maturity as much as other aspects.

There is no rule of course, but if a person dates someone that is a lot younger, it could seem like they are taking advantage of them and they possibly are, even if it unintended.

I personally feel like the 80% younger person has a pretty low possibility to be close to your equal in maturity, but even bellow 90% can be suspicious.
I guess you can see connection between that an IQ, as I believed that once IQ was defined as a maturity compared to age, where 100 means 1:1 ratio.
There is only a very small percent of population bellow 80 percent of IQ and they are classified as having border-line deficiency.

The point is that it isn't really fair to the less mature person to be in a relationship with someone quite more mature then them.
The wiser person in a relationship could quite possibly take charge and get his/her way even in the best intention.
Certain equality between the couple seems to be quite necessary, but there are of course many people in many different situation in their life, so making any quick judgments could be unfair.