If you have kept a journal for a decent period if time, looking back at it, it will reveal far more than your actual words. Of course it does depend on the quality and quantity of your journaling - but on the whole, our journals do tell us much about us. It is a good tool for analysing habits and behaviour.
When I look back at my journals, I see all different goals I have focused on, various things I have obsessed about, the progress I have made, failures faced. Looking back at it - my journals not only tell me what I have tried to achieve, but they tell me why I have failed or succeeded. I don't sit there writing theories of reasons. I simply record my obsession, what I am doing, what I wish to achieve. This applies not only to goals, but to your general life. Perhaps looking at your teenage journals, you might find yourself obsessed with boys/girls, in your middle age with work/family, loneliness, illness - anything. How did you cope with trials in life? How did you appreciate the joys?
When you look at your journals with the purpose of analysing your life, and more importantly, your self, you can see the patterns of behaviour. Is there a habit you need to change? A fear you need to overcome? Is there something that is consistently holding you back?
All the answers there - if only you have the patience and the courage to look for them.
And if you haven't kept a journal for a consistent period of time, it is never too late to start.