In a professional medical sense it could be used as a stage i guess?!
My mother, who is retired now, used to teach special needs adults with the primary purpose of enabling them to lead full and functional lives. A vast proportion of them ended up with their own home, a partner to share life with, a full time job, friends etc...they are functional adults even with their "disabilities".
I can't help but find it funny when an alcoholic/adult labels themselves "functional" as if it something to be proud of...the average IQ of my mother's "pupils" was around the 70 mark, for them it is a real achievement to be able to live functional adult lives...for an adult of average IQ and ability, even with addition issues, personally i don't find it at all impressive but everyone's got their own opinion on this one and that's ok...
Using the word functional is just another string to the bow of the addicts/co-dependents denial:-)