I, for one, certainly realise how old Kate is compared with Philip. She is 32 and he is 93, and I think the relative age of the two people in question is precisely the reason for the criticism of her lack of work.
Yes, she's breeding at the moment, but pregnancy, of itself, is not an illness, and many women hold down responsible full-time jobs throughout their pregnancies. She's had some pregnancy-related ill-health and no-one expects her to do anything when she can't hold down food and is likely to be spending half the day vomiting, but she seems to be over that now. And she has a full time nanny to help with George and I'm betting she has help with the housework and cooking, too, so it's not as though she is a full time housewife and mother with no-one to help her and free her for other things. So why isn't she doing, and - just as importantly - being seen to be doing, more?
Oh, and 32 is not young. For some reason there is a tendency to refer to this 32 year old adult as a "young woman", which I can only assume is an attempt to excuse her from not doing more than she is. However she has had more than 10 years to get used to the idea of being a Royal wife, and she's been one for nearly four year now. So why isn't she performing more royal engagements and doing more charitable work. She's really not setting a good example for the (truly) young women who look up to her.