I agree with AIMD that Dougalthesyrianhamster's comment was unnecessary and quite bizarrely harsh. Also illiterate with the smug "going by your username you're only 27" when literally the first words in the opening paragraph from OP are "I'm 28."
I think it's pretty obvious that owning a house per se is not necessarily any more mature, but that (in the same way as being a long-term renter is) it is one indication of someone that is sorted and grown-up, in that you have had to reached a certain level of financial competence, organised yourself to keep the home maintained, pay bills on time, sort insurance etc. every year, all that boring adult stuff which wouldn't apply to other 28 year olds who could still live with their parents or in a flatshare that they swap every few months. Of course they could be more mature in other ways. OP was just given some examples to show that she was living a relatively responsible, settled life and thus there was no need to patronise her.
I disagree somewhat with the other comments saying you shouldn't stop your other friends spending time with this woman - firstly I think you should check to see if they actually all do like her. If they are all like you and don't like her but don't want to be the one to say first then it does seem ridiculous for you all to spend time with someone most of you dislike. Even if they don't mind her, surely they are closer friends to you - if one of my best friends was being repeatedly put down and patronised by someone who wasn't much more than an acquaintance I wouldn't want to keep hanging round with the acquaintance at the expense of my best friend, even if I personally didn't have any issues with them.
Finally, friendship groups aren't always stagnant and it's good to make new friends etc. - but you are already a group of 8 - if each of you decided to bring just one extra person along you'd be a gang of at least sixteen and then the dynamic you have now would have to change - why does your original friend have the right to impose someone upon you all?
I'm not saying nobody new should ever join a group but as a bare minimum they should be nice to the original members!