You must be at least 21 years old for domestic and international adoptions. In the US, you will not have any age limitation for adopting a child. This means, if you are 21 years or older you can adopt a child.
To adopt a child in the US, you must have stable medical health. Moreover, you also must provide a physician's letter if either of the adoptive parents is suffering from serious ailments.
If you are seeking private newborn adoption in the US, then you’ll have to provide marital evidence. On the other hand, foster adoptions do not require any marital requirements.
Before you seek to adopt a child in the US, both state and FBI shall conduct a home study to determine whether you’re capable of adopting a child.