Please note that I did not say anything about halachic or hashkafic differences per se’. What I said is that the two are very different. My point is that when a 20 year old girl talks to a boy something very different is going on than when a girl in her 30’s talks to a boy (at least in most cases). You simply can’t compare a 20 year old girl to a girl in her 30’s.
There are other differences in this case, and perhaps it is those differences that are more important in this particular case, but I felt that emphasizing this difference would be understood better by the person to whom the comment was directed.
If you still feel it is hard to understand, consider the following points:
1. The Gedolim encourage girls in their 30’s to talk to boys. I know this for a fact. I may be wrong, but I am not sure they would do the same regarding a 20 year old girl.
2. Have you ever seen a 20 year old talking to a boy? Have you ever seen a girl in her 30’s talking to a boy? Can’t you see the difference? (again, there are exceptions, but I am talking about the majority). That is one of the reasons why I am assuming that most Gedolim would not say that 20 year old girls should talk to boys.