Aaaah! I kept a diary from age 15 onwards, but it drifted off when I had kids at 28.. somewhat - I still kept one sporadically, whilst pregnant and in the early years...reading the early diaries - I used to cringe - but now am far enough from 15 to roll my eyes and think 'Oh bless!' (am 42 now)
I have diaries from when I was in labour with the children - one is punctuated by contraction timings, and (in early labour) "am trying to read 'Dracula' but have read the same page 4 times now, cos the contractions keep interrupting and I ...Arrghh!" (7 mins apart, 45 seconds long) " Blimey - I remember how much it hurts now - am going for a bath..."
The early diaries are entertaining, and although they are filled with " My mum hates me, and wishes I had never been born!" (she was fine, and just dealing with a teenager) "Had a great day at college today!" (A boy I liked noticed me) and "Gave him something to think about" (Karate competition, kicked someone very lightly on the head and won the fight..) "I need some new clothes!" (I didn't) "Where is my life going - am adrift!" (existential teenage angst!)
I also have a set of notes me and me mate passed to each other, about period pains, and wanting to emigrate! It was somewhat in sympathy on my part, cos I didn't start my periods till I was 16... 
Bloody hell! now I have my own teenager - a 14 yr old boy! who is taller then me, with hairy legs,who has to shave once a week! But he still talks to me - not about absolutely everything - that would be unnatural - at 14 - you can't hope to be privvy to their very innermost thoughts - just to be there for them - but he still tells me stuff, about what he thinks and feels, and occasionally - with studied indifference - he indirectly asks for advice. I have to pretend I'm not giving advice, and talk in general terms - It makes me smile!