My step daughter is 23 years old and has asked if she can legally hyphenate her last name to include mine. (I am married to her mother). Her biological father has not been in her life for several years, but she does carry his last name. She has no plans of getting married and if she does, she will keep her hyphenated name as a prefix to her newly married name. Since she is over 18 years old, do we still need consent from her father? Also, how would we go about making this change legally?

Personally I think it's a bad idea... and its going to create a huge amount of problems in the future... from SSI, to bank accounts to drivers licenses... if she ever tries to get a passport or security clearance... and there are other things that probibly didn't even come to mind while I was writing this...including even getting on an airplane.

No consent is needed. But to make this a legal change, she has to go to court.

On the other hand, she can use whatever name she wants (as long as there is no attempt to defraud) wherever a legal name is not required.