In this sense, Satya, I would argue to state that personality traits, for the most part, are defined from birth and if we are comparing personality and homosexuality, if they follow the same lines, conclusively we can say both originate at birth.
You can see a difference between even infants in the way they interact with the world. Some cry 24-7, there have been others, who do not cry at all. You can only see these develop as personality traits as they get older because it is easier to spot; there are, in obsevational senses, always clues of behavior of a child's personality since birth, but it just isn't plainly obvious. Which could be why people are prone to believe that adolescence defines personality, because at that period of time the personality traits become much firmer and the easiest to spot. All of this would apply to homosexuality, too, if we were to go by your model.