To Kill a Mocking Bird is an old movie, where segregation is taken for granted, that may be my only reserve when showing it to someone under 15 year-olds, it would be great if an informed adult would be present to answer questions.
Atticus is an example of integrity for all the generations to come, his children take a stand on issues that are beyond their apparent reach, and I think that is an often forgotten but very important part of the movie.
A murder takes place and it is agreed that the person who gets murdered is so bad that the person who did the killing should not be brought to trial. This is a very risky proposition for a young person to digest by him or her self.
In the whole, I think the movie will always be "relevant", but adult guidance is strongly advised.