All adults should have access to their history, whether adopted or not. This is a matter of civil rights: knowing your history, where you come from, medical issues in your family, etc.
In the past, closed adoptions were the norm, and the birthparents did not know who the adoptive parents were. Because this was the norm, the idea of not allowing contact between the adoptee and their birthparents also became standard wisdom.
This has all changed with the advent of open adoption. Now the birthparents and the adoptive parents know each other and share identifying information. In this day and age there is no ethical or legal basis for discriminating against adult adoptees. They should be given access to their birth records.