When you look back at them it actually brings back all the feelings you had at the time when you were writing them. When you wrote them crying, happy, excited, dreamingly or just to kill some time and make the diary look prettier.
They are great to keep stored forever and maybe even show your kids and they can see how their mum once had the same teenage problems and feelings, although some children find it hard to believe that parents actually used to be teenages and kids.
I think it is good to have diaries forever, no matter what age or gender, diaries are a way to express feelings that sometimes we just feel like telling ourselves and that's that. No one else can read them, unless they go through your things, and you can feel whatever emotion you have in you in the privacy of you and your best friend, the 'secret diary'.
To make them look even prettier, you can add pictures and drawings, even jot down things you have to do or buy, uni marks, job pays and letters from your loved ones to make it like a scrap book sort of thing.
Secret diaries aren't just for young teenagers, they are for all those people who love writing and who want to express their feelings to themselves in a calm and artistic way.