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.
/ramblings
Good question. Basically, Se deals with savoring the moment, having fun, and acting immediately. Together, all of these traits are definitely more youthful traits (they are more common in young people than they are in old people). Young people seem, in general, to be more fun-loving and in-the-present.
Si has to do more with oldness, because of simply what it is. Si has a lot to do with looking back at the past for reference. If one is very young, they don't have much to look back to. Older people have a lot to look back on and reference, simply because of their old age, and Si rewards experience in a significant way.
Old people are supposed to be more work-oriented than children. Si is more common with those who plan things and who are meticulous and organized, whereas Se, as mentioned earlier, has to do more with enjoying the present moment, something which, I would say, children do better than old people.