Can I (18 years old) be gifted an Ar15 (rifle) that has been built from a lower?

Im unsure with how the process works with transferring an ar rifle to my name that has been built from a ar lower that was bought as an “other”

I say this because you have to be 18 to buy a complete rifle and be 21 to buy a lower receiver.

After a receiver is built into a rifle is it considered a complete rifle to transfer?

Thanks