One may think that everybody went over it several times,but I think that is good to recieve new opinions,so we can get closer to a consensus on this topic.
In my humble opinion,the characters age is the same as they seem to be in the movies,but in a relative way.
It was stated by the Show's Creator that they are beetween 12 and 18.I believe we have to take it as a psychological age.
They act like young (sometimes inmature) people while being eager of their destiny.They have resposabilities of their own,but they don´t show them playing some common important roles,that someone would have in a life project.We don't see any of the Mane 6 having a child,for example.IMHO,the Show´s Creator wanted the main characters to be as young and independent,but not compromised as possible.
Then we have the fact of their society being totally diferent.Perhaps in theirs,with everypony knowing and working with their talents,they don´t need further education,gaining advanced knowledge,or other type of preparation,so they begin to have jobs younger,and get independent younger.So,using this logic,they can easly be the movies age with no bigger problems.
The fact of some characters being thousands of years old,is caused by magic,so,in a world were magic doesn't exist in normal situations,they need to show their real age.This also happens with the social roles,they try to make them look realistic as possible.
Finally,I want to apologyse if I sound disrespectful while exposing my thoughts.I respect all of the opinions about this topic.And please,feel free to give me yours.