Define "mature."
Windows Phone isn't iOS or Android. Some claim that a "mature" OS requires 750,000 apps, or the ability to change the system font to Comic Sans and install a custom ROM, or the ability to run some proprietary app only available on one platform.
BlackBerry users would say that a phone without BBM and the ability to run for three days with push e-mail on a single charge isn't "mature."
webOS users would argue that Android, iOS and WP are all "primitive" and "not ready for prime time" due to their multitasking limitations and UIs that don't facilitate active multitasking.
Symbian users would point out that Symbian is more "mature" than Android, iOS or WP because it has been around longer, runs on more devices, and can last days with just one battery charge (while having a library of hundreds of thousands of applications, many of which have evolved over a decade or longer on the platform). They could also point to the Nokia 808 Pureview and say that all cameras other than Symbian's are obsolete and totally inadequate, thus not "mature and ready for prime time."
Palm OS users would say that the PIM components of Windows Phone, iOS and Android are inadequate and immature, and thus usability is limited.
Windows Phone users would say that iOS has poor integration with Windows on the desktop/laptop/tablet, a lack of serious productivity applications, and an outdated interface, while Android is a battery-sucking, laggy malware pit with serious security and usability deficiencies. WP users would also note that iOS and Android are poor at cross-data and cross-account integration, requiring users to open separate mail applications for multiple mail accounts and not cross-linking basic contacts in a useful way.
They're all right, in a way. And they're all wrong, in a way. There is no "one way to be." Choose what works for you, and run with it. Try out new things. And keep an open mind. If Windows Phone was just like Android or iOS and did everything the way they do (or the way they try to define "mature"), there'd be no point to WP. WP does things differently by design, and many of the so-called "features" (like confusing three, four, or five different volume settings) aren't all that useful.