When should a child know they're adoptees?

Long ago, a 4 month-old baby girl was found on an abandoned car seat right on the side of the road. My friend was then in her early 30's at the time.

She took the baby girl, reported the incident and successfully went through the adoption forms and procedures. The girl is now 15 years old and doesn't know the truth yet. She calls her mom. My friend is worried that she might find out one day. She doesn't want to hurt her daughter (to her... she is her daughter; even if not biological) and doesn't know how she'll react.

If you were abandoned but then adopted by the kind woman that found you... when would you want to know this? When should adoptees be told the truth?
When should a child know they're adoptees?
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