A.-
She's not too young, but keep it simple. Both my children are adopted and were adopted at birth. In fact, my husband and I were both there at the hospital when they were born and brought them home from the hospital. They have always known they were adopted.
I would offer 2 thoughts - Keep it simple and no big deal. Just tell her that your husband is her Dad legally and in his heart and soul. Tell her that her biological father wasn't able to be around and wasn't ready to be a Dad, so you found a wonderful man who loved you all and was ready.
DO NOT go into the issues with her biological father - that can come when she's older.
My second thought would be: if she wants to meet him, tell her you will help her find him when she's older --like 18 or so. That's what our agreement is with our childrens birthparents and that out daughter accepts that answer. She'll be old enough to handle whatever the situation is like with him then, and you won't have to worry about it.
God bless you all -
C.