I adopted my child when she was 3 months old and she will turn 2 next month. When do you think I should tell my daughter that she is adopted and how should I do it?

My first cousin is an adopted child. And she has no idea up until now. My auntie and uncle got her a day after she was born and she doesn't know until now (she's 21 years old). They said that they will let her know once she's done schooling so that if anything goes bad like she'll decide to leave them - they're already "done" with their primary obligations to her and that she's mature enough to think things over on her own. But having able to live "closer" to her.. I can say that she's been a happy, normal child. Though from time to time people compare how she looks like from us, because it's evident that she got fairer complexion and has chinkier eyes than all of us in the family - thus makes her browse rise. Hmm.. Can say that if I were in her shoes I won't feel bad about it 'cause she was loved very much and was treated normal. I know that it's her right to know the truth someday and believe that there will be a part of her that will really make her probe who her real parents are - cause truths makes her who she really is.