My mother was diagnosed in her early twenties (22?) and got very very sick before diagnosis (lots of embarrassed medical family members over that). She had no diabetic family members at the time. As a consequence I was diagnosed very early (just two nights of waking up to ask for a drink of water!) at 8. None of my siblings or children have diabetes, though I used to test my childrens' urine whenever they complained of excessive thirst. (I was told that the children of a T1 have a 1 in 30 chance of developing it, but this was a while ago.)
Having said that, my father and both parents-in-law developed type 2, so my children will have to watch their lifestyle/diet to avoid that in their future.
My understanding was that T1 has a small genetic element and there's pretty well nothing you can do to avoid it, while T2 has a large genetic/lifestyle element.