"There's only so far you can take a relationship before you got to get into things that are too serious or over the top. I'm 20 years old. I was in a relationship with a girl I love for three years. Where do you go after three years? Then you've got to start thinking about other things, and I'm too young to think about those things"

Shia LaBeouf is suggesting that relationships can just presume prior to they become too severe or over the top. He is referring to his own experience of being in a relationship with a lady he likes for 3 years. He is indicating that after 3 years, it is time to start thinking of other things, such as marriage or having kids, which he is not ready for yet as he is only 20 years old. He is suggesting that it is necessary to recognize when a relationship has reached its limit and to not press it further than it should go. He is also emphasizing the importance of being aware of one's own age and maturity level when thinking about the next actions in a relationship.