Defo mid to late teens. 16 to 19 was a dreadful time... They are trying to push boundaries and think they're adults and can do what they want, and are moody and sullen and uncommunicative, and think they are right about everything.
When mine was 17-18, we were the last on her list of life's priorities. She literally spent 50 quid a time for Christmas on her boyfriend of the time, and BFF's and we got sod-all as she had run out of money. She also wasn't arsed with mother's day and father's day, and acted like we were a nuisance. Hated those 3 years.
We were quite protective of her, but by that I mean not allowing her to go stay with people she only knew from the internet, who lived 100 miles away, and who she had only come into contact with a month ago! We also would not let her go to Glasonbury when she was 15 with a bunch of 18-20 year olds she knew from twitter.
She said (when she went to uni,) 'I am leaving and won't be back, ever, so you may as well rent my room out if you want.' It was hurtful and upsetting, but she had clearly felt trapped by the constraints of us trying to protect her. She went to a uni 350 miles away and said she will live in the place when she leaves. Again, I was a bit fed up and blue. We let her go though, we helped her move, and we were there when she needed it. It was a case of 'if you love someone set them free.'
5 years on, and she lives 15 miles from us with her boyfriend, we see each other once or twice a week, and we talk all the time on the phone/facebook/twitter. She has grown up and matured and can now drive, she has a great professional career, and we are all good friends (me, her, her dad, and her boyfriend.)
But those 3 years were vile. I would hate to go through that again.
No other time of her life was bad. I even enjoyed the first 3-6 months. (Apart from a bit of sleep deprivation!)