What if the bio mom doesn’t consent to a stepparent adoption?

My stepdaughter is 19 years old and I’ve been in her life since she was 4. Her father did drugs and was in and out of her life. At this point, we have no idea where he even is, could be in jail. My wife and I have talked for years about me adopting my stepdaughter. Because her bio dad always said he’d fight it, we never pursued it. Now that he’s been out of the picture for several years, we are considering it again, and my stepdaughter wants the adoption. Does it matter if we can’t find her father to see if he’ll consent or, if we do locate him, what happens if he objects? Does it make a difference that she’s now over 18?