Se seems to be very much associated with youth and Si with oldness.
Why is that?
It seems like as a society Se perception is something we're supposed to grow out of.
This perception is something I've very much encountered myself.
I used to go around saying I was disgusted by Se, but when I really examined it, I wasn't disgusted by the actual behavior, so much as the fact that the behavior had been associated by society with childish behavior.
So essentially, if Se bothers me, it's because I've learned that it's childish and unacceptable, not because of anything inherently wrong with it.
I think the reason it's discouraged is because most people (except INJs) tend to naturally operate in an Se mode regardless of deterrents, so discouraging Se and encouraging Te and Fe is necessary to get people to focus on following rules and the future at all.
[YOUTUBE="http://www.youtube.com/watch?v=dJrEdnEZY-8"]Silly SPs, Se is for kids. [/YOUTUBE]
I think Ne can be perceived in this way too. It's probably because of those activities typically associated with childhood (physical thrills, exploring, playing etc.) and those associated with adulthood (record-keeping, organising, scheduling etc.).
I know that as a child, my Se was pretty much unused. It's something I've had to work at and still kind of drains me. I've found it to be a function you're kind of forced to use in the real world, so I think I've acquired more of it with age. So I guess I tend to associate it more with a kind of 'street smartness' and awareness.