What do you expect? 20+ year old girls composing music from A to Z?
Absolutely. It's not unheard of at all. Age and gender is no measure for talent.
Yui was still in her teens when she started writing her own material - both music and lyrics - and was 18 when her debut album was released. Same for Tsuji Shion and Miwa. Not to mention the countless male bands and singer-songwriters who have been composing their own material whilst still in their teens - Stevie Wonder was writing No 1 hits for himself by the age of 16 and he's been blind since birth! Can't get a bigger handicap than that.
As much as I love SCANDAL they really should be composing their own music by now and being 20 and female is not an excuse at all. The fact they have always written their own lyrics is a good start but with a third album looming now they should be relying less on other songwriters for the music. I don't mind if they continue to keep their own compositions to B-sides and album tracks for the time being but sooner or later they have to start being confident enough to write a single of their own other wise they are never going to fully progress both musically and personally as a band.
They've proven themselves as musicians and performers now they have complete the hat-trick and be songwriters too. And I believe they can do it.