All three of my children process thier adoption story differently, even though they are siblings and they have fairly regular contact with their biological family.
Sissy does it in spurts. She'll have a period in which she asks lots of questions but won't listen to the answers. Then she'll leave it alone for a time and act like it's no big deal. Of course, her story comes with sorrow since she was the only one of the three that was in their birthmother's care, the impetus for her eventual RADs diagnosis.
Aspie Boy doesn't really care. Or at least he doesn't give the appearance of caring. But it begs the question, with an IQ just two derivations above MR (mental retardation), just how much of those more complex and abstract concepts about family is he willing to understand or even capable of understanding? When he asks, it's usually the same question, "but WHY isn't my birthmom my mom?" which is always answered in the same way, addressing birthmom's specific needs that prevented her from parenting.
But Wonder Girl has never really asked, until recently. Last night's conversation after bathtime was too adorable to not post it.
I was toweling her off and she said, "Mom, hey, look! I'm going to get my boobies soon!"
I just smiled and said, "Well, usually breasts don't develop until a girl is older than 5."
"Oh." she said. Then as an afterthought, "But my baby is growing in my tummy right now!"
"Why do you say that?" I asked.
"Because it is! Girls make babies in their tummies!"
"when they are grown up and married to a man, yes." 
"oh. right. But mom? When does a baby get in there? when did I get in YOUR tummy?"
"When a mommy and a daddy love each other, that's when they can make a baby together. And remember? You didn't grow in my tummy."
"Oh, yeah. I forgot. Why didn't I grow in your tummy?"
"Because my baby maker, it's called a uterus, is broken."
"So you can't make babies?"
She put one hand on my shoulder and hung her head, shaking it, "That's too bad." Then she bit her lip. "sorry, mom."
That was probably The.Nicest.Thing. anyone has ever said to me about my infertilty. "Well, it's ok. I still got to be a mom."
"Yeah! and one day I'm going to be a mommy, just like you!" Then her face went dark and fear was in her eyes. "MOM! what if my baby maker is broken?!"
"Then if you still really want to be a mommy, you can adopt children like I did." 
I finished dressing her and neither one of us said anything for awhile. Then she said, "Mom? If Sissy, Aspie Boy and I grew in your tummy, would we still be Sissy, Aspie Boy and Wonder Girl?"
A question I ask myself all the time as I process my infertility grief. How would my life be different if I had biological children, who would my children be? And I always come back to the same thought, but then I wouldn't have shared my life with Sissy, Aspie Boy and Wonder Girl and they are too wonderful to have never loved! So I answered Wonder Girl the same way I answer my own thoughts, "I think so, honey, I really think I still would have been the Mommy to Sissy, Aspie Boy and You. One way or another, I was meant to be your mommy and you guys were meant to be my children."
She liked my answer and smiled her approval. "Yeah, cuz that would be weird if I was someone else because my birthmom gave me my name and then who would I be?"
"Actually, I gave you your name. And Aspie Boy's and Sissy's middle name. Birthmom only gave Sissy her first name."
"Really? YOU picked my name?"
"Cool!" and she hugged me, hard, then grabbed her toothbrush to start brushing but just as it entered her mouth she stopped and said, "I want you to do it... please."
So I took her toothbrush and lovingly made all of her tiny baby teeth as clean as I could and we giggled at each other because she kept making faces with her eyes and eyebrows while I brushed.
And that's how Wonder Girl processes her adoption story. I still havent' made their stories into little books for them. One day I'll get to it.
 yes,yes, i know, i told a little lie about how babies get here. At 5, I didn't think she was ready to hear the lesson about HOW babies are made and all of the various ways families are formed outside of the biblical construct God intended for families. And PLEASE don't start commenting about all of the wonderful families that are created outside of that construct because i KNOW that, I'm not dissing all of those wonderful stories. But she's 5. And it's hard enough to process one's inception within the normal mom/dad/marriage/make a baby by sexual intercourse concept let alone through adoption when a birthmother is deemed unfit by courts, the same birthmother that unwittingly harmed an older sibling! Baby steps. Baby steps. One day we'll cross all of those bridges.
 this isn't actually the full definition of my opinion about why someone would choose to adopt. I wholly subscribe to the belief that adoption is NEVER a plan B to parenting, adoption should be something a person actively desires because they feel compelled to help a child that is parentless or in need, or both. I regularly tell woman that come to me about their infertility, "don't choose adoption until you've grieved your unborn children and are prepared to love another woman's child and have understood and are prepared to accept all of the nuances that go with that child and his/her biological history and family. It's a different path to parenting, not an alternative to bearing biological children, period.