My “comeback” was equal levels rude?

Is it an island nation perchance? Half my family come from the island I think you may mean and I've endured very similar comments particularly from one uncle who basically told me I was 'on the shelf'.
OP, I completely applaud your excellent retort. It was justified.

Remember ladies, we must be polite and nice at all times otherwise all we are good for is fucking

These people are regularly feeding and entertaining OP at their home to the extent OP is pissed off she is no longer welcome back.

That sounds lovely chattel duty, where do I sign up.

So did mine! OP, you need to find a way to make your displeasure felt whilst remaining polite.

She was rude and your reply was rude, funny but still rude. Good for you not being a doormat but she doesn’t have to be one either. You needed her hospitality which she has withdrawn, presumably she needs nothing from you.
There are better ways to deal with these kind of comments than lowering yourself to their level. ‘Old enough, you?’ is what I usually say. Or ‘in my culture we consider that question rude’. It’s much harder for people to then blame you for also being rude.

Yikes, he’ll never marry you… (if that’s even what you want) his family will hate you forever now!

She can go around calling people fat all she likes but it’s hardly a ‘witty joke’. Just a mean response to what she percieved as being called old. You can hardly then moan that the woman and family are now upset with you.

NO ONE in bf’s family (his parents and siblings) thinks what I did was right. My mum said it was totally fair to highlight the Aunt’s rudeness.

This speaks volumes really.

You were ruder than her.
Asking someone, particularly someone younger than you, their age can be rude or neutral. You were assuming rudeness because of your dislike for her. You could have answered in a way that showed you found her rude or not answered. Asking anyone their BMI (outside of a medical setting) is always rude and particularly so to an older relative.
I wouldn't expect her to include you in any future gatherings, seeing as you dislike her that shouldn't bother you too much.
I would question whether your age and "being past it" is something that bothers you - if a boyfriend's older relative asked my age, I may have considered the question strange and maybe rude or intrusive. But it wouldn't have upset me or made me want to insult them.

I would be more concerned about what kind of family any future children of mine would be born into. Worse still, if the relationship broke down and the bf/husband got 50/50 custody of them. I'd just move on tbh, and as a PP said, find a less shitty family.

Have you not heard of the phrase “when they go low, we go high”?

The correct response range would be something around “why do you need to know that?” “I really can’t remember” “ that’s not a question you ask a lady”

You could have been assertive but instead you were aggressive to someone in their own home.

She was cheeky.

You were rude.

There's nothing weird about asking someone their age. You were really weird and rude about it

Maybe the OP thinks more of herself and has a moral code that sits outside of whether or not she wants to be seen as appropriate chattel for some man and his family.

Didn't say they were. I just said what some of them would do and that would be to keep the sexual relationship going as long as they could before ultimately cutting them off altogether.

She was possibly going to be rude, through ignorance. You were rude with the full knowledge of what you were doing. You meant to hurt and humiliate, all you were going to feel from her comments was annoyance. I think she sounds awful but on this occasion you were worse.

Then don’t worry about being excluded.

Surely what you said was a pointed joke. You weren’t expecting an answer.

Gosh you were horrible. What a disastrously touchy response.

Yet on another post where the OP was offended by 'jokey' comments about her height from her in-laws this is precisely how she was being told to respond! Make your minds up folks, if personal comments are made to/about me then I consider the gloves are off!

There's nothing unhinged about it. They may be my family but they are still men.

All you had to say was “I don’t think that’s an appropriate question” and you’d have called her out but kept the moral and social high ground.

Well if the aunt was implying the OP is old then what’s wrong with the OP implying the aunt is fat?

Use your imagination?

I’ve never really understood this thing about age. Why is it rude to ask someone’s age? That only works if you think there’s something bad about being older – the OP used the phrase ‘past it’. ‘On the shelf’ is another one. Older people in my culture can sometimes be seen as invalid, not worth listening to but I don’t buy into that and I’m always very happy to share my age.

Fat shaming is a nasty thing to do though.