1) Simple symphonies are not complex. No matter how many instruments a song has, they tend to follow the same tempo, same melody, same technique, etc.. with the exception of the one or two lead instruments.
2) That's easy for people of any age to understand, NOW, I'm not saying enjoy, since one can understand rap music, decipher it, follow it never getting lost, etc.. but not enjoy it. Do you see where I'm standing now?
3) People who have been training their ears to complicated music tend to enjoy it more than any person who has never trained their ears. You can pick that up at any age whatsoever, but if early the better.
4) The reason why many kids don't follow classical might be because they are not willing to master their ear or are just don't know that you have to do that in order to at least enjoy it. Other reason is because kids want something more exciting that makes them jump or talks to them (The power Rangers songs, Barney, Pokemon with its "You gotta catch them all", etc..). Remember that kids are mostly hyper, with the exception of some kids who prefer something calm. It is not till later where they outgrow simplicity and look for something a lil bit more complex.
But hey nothing in here is written on stone.