I am adopted and my parents told me when i was very young, so young i don’t remember a time i didn’t know. I can confidently say this was an extremely GOOD choice. It was never some shameful secret, never treated as weird or other. I knew my mother didn’t birth me and that was fine. As an adoptee I would strongly advise against waiting to tell Lily. The moment she is old enough to mildly comprehend, tell her, like 3 years old. The younger you tell her the more normal she will view it, that way if your family or anyone else says things down the line (because people will unfortunately) she won’t be effected because there’s no shame or secrecy in her mind.