If it needs to be spread out over the kanji by their specific reading, then it would be おとな・げ, because おとな is a jukujikun where you can't assign any of the parts of the reading to the individual kanji, they all are the reading for 大人 as a whole.
This kind of thing is something the lone creator of the site would have to adjust manually. So it may not be something he's able to get to soon.
JMdictDB does not incorporate furigana and I also read in a discussion about this that it is not a feature to be integrated into the Database anytime soon. However, there seems to be a seperate repository for the furigana only, but I don't know much about it.
https://github.com/Doublevil/JmdictFurigana
In this case it's as Leebo said, おとな is a 熟字訓 reading, i.e. a reading that corresponds to the kanji's meaning while ignoring the kanji's readings. So it is impossible to seperate furigana here.
Furigana Error
Reading is currently broken up as お・と・なげ when it should be お・とな・げ
(As far as I can tell, JMdict doesn't separate its readings into parts so there's nothing that can be corrected over there?)