It's no use trying to reason with family members who do this, trust me, I've tried numerous times and it's not worth the effort.
In my case, it's a combination of having been typecast as the little one (I'm in my late thirties, but that apparently makes no difference) and therefore naive and easy to manipulate. I was naive - when I was little, as in a child - so yes, I was young once, as everyone has been, but that was then and this is now, many years on, and besides, I have never ever been easy to manipulate, quite the contrary, so I don't know where that part came from.
Still, that is my role in life according to my mum, which comes in handy when I do anything that she doesn't like: of course that is not me, it was somebody else's idea, and consequently she needs to bring it up (over and over and over) to open my eyes. Incessantly. Over the years. Because she loves me (which she does, I have ample proof of it). And apparently I never learn. So ever since I deviated from the path that she thought ideal for me, firstly by choosing a DH that she wouldn't have chosen for me (we have a good life together and I'm happy but what do I know, right?) and then by going freelance after having a very well paid corporate job (not my choice, I was made redundant, but it has turned up really well and now I earn just as much with far more flexibility), she needs to remind me how naive and prone to manipulation I am, and also "make sure" that I save money and don't spend it foolishly, or something to the tune. By "making sure" I mean repeating it endelessly, bringing it up every time that money is mentioned even in passing and insisting even after I assure her that I manage my money very well, thank you (because of course I don't, obviously).
To put this into context, I have been financially independent ever since I finished my education, which is a step higher than my sister's (both university educated, me a postgraduate degree) but no, my sister is the responsible, savvy one; she is also, and this is no contradiction in my mum's mind, really hard-done-by with her middling income, bad partnership and rather difficult children. Oh, but she works so hard, and takes them here and there, she's sooo tired from doing everything herself (because her DP is a shit father and partner). And she still cannot make ends meet; again, nothing she can do about it, such as dump the DP and her dead-end job and get herself a better-paying job for which she is amply qualified, oh no, she's a MOTHER and everything is so difficult and complicated and no one ever worked this hard so she needs to be helped every month by my mum, a pensioner herself. Because, you know, it's all so hard for her.
I'm a mother too, and work full time with no back-up as I'm self-employed in a highly qualified capacity, but somehow that doesn't register in my mum's worldview. In her mind (and my sister's) I'm very silly and naive and cannot stand on my own two feet... See, I cannot even handle my business properly and need to be told so, in spite of being very successful at it. That is entirely irrelevant, you see, 'cause I'm just a silly little girl.
Facts cannot stand in the way of preconceptions, it's as simple as that. In my mum's and my sister's mind, I'm stupid, weak and naive, and my sister knows her way around the world, despite the fact that she actually, demonstrably, does not and that by the looks of it, she will never stand on her own two feet until my mum either dies or runs out of money to subsidise her. And that fact - being subsidised to maintain her fantasy that she is, indeed, a functioning adult - cannot be mentioned to her, or to my mum, because it's not her fault. A highly qualified woman in her early forties with a partner and a job needs her mum's help every single month to make ends meet but there's nothing she can do about it and no reason why this should be changed or mentioned at all. Right.
Rant over. Now, to answer the OP's question, YANBU to be pissed off. It's belittling and demeaning. However, don't expect it to change. As I said, facts cannot stand in the way of a long-held preconception. You either learn to live with the infuriating reality or you go LC. I'm LC with my sister for this and other reasons, and just take a big sigh of resignation before and after I see my mum, because I actually do love her and on the whole she is a good mother who brought me up really well (but cannot see me as an adult and never will). It's just the way it is, sad as it may be.