She's hardly young and has been around the Royals for 15 years. She's only a couple of years younger than Diana was when she died but has done nothing of note and never bothered using her education or worked in any kind of career.
I think that's kind of the point of why the Royal Family are happy with her position and she fits in well. Because she is the anti-Diana. After all, Diana wasn't that great for the Royal Family was she? I really don't see why they would want somebody who emulated her.
I think Kate just kind of encapsulates the fact that William has learnt from the mistakes of the previous generation. Diana was mercurial, unpredictable, tempestuous and impulsive. She overshadowed the Royal Family, eclipsed them and often behaved in ways which brought them to the brink of destruction. She courted the press and publicity and routinely manipulated the press to her own ends. None of this was good for the Royals.
Kate seems to be very stable, predictable, reliable, stoic, doesn't rock the boat, doesn't court the press outside the official press calls and there don't seem to be many leaks about her family so not spining about herself to the press. She may not be show boating about big causes about AIDS and land mines, but by the same token she is not overshadowing things like the Jubilee celebrations or the Prince's Trust or Prince Harry's work in Lesotho and giving the impression that she is the only Royal who is caring.
Diana said a lot and it caused a lot of trouble.
Kate has always kept her mouth shut (much like the late Queen Mother).
Fergie and Andrew's marriage largely crumbled because he was away in the Navy and she was not allowed to go and live on a base like a normal navy wife so they never really saw each other or had a proper married life and she turned to other people.
Kate isn't really taking anything else on and has been following William round, living with him at base and having a normal married life.
Feries (post marriage) has embarrassed the palace several times with her commercial ventures. Sophie Wessex had several scandals over her PR company including a massive one with the fake sheik where she appeared to cash in on her Royal connections.
Kate not working means she can't be criticised for doing things in business in which she profits from her Royal connections.
She might be dull. She does just turn up to a few ribbon cuttings and services, look pretty and go home to look after the kids. But from the Royal family's point of view she is pretty damn perfect.
We might be moaning that she's lazy, but if she was killed in a car accident tomorrow I don't think it would come close to bringing down the monarchy...