This is such a tough spot to be in, I think many many people have strained relationships with their parents when trying to establish independence.
It can be very difficult for parents to allow or accept that their children have become adults, for a host of reasons... powerlessness to protect, fears that you will grow apart from them, loneliness. Depending on their experiences it can lead to controlling behaviour but I think it's really useful to remember that it comes from a place of fear or uncertainty.
It's very very feelings based! So I wouldn't recommend basing your response on any scientific arguments about maturity, because I strongly doubt that science has anything to do with where her words are coming from.
Feeling controlled can make us depressed, and angry and some of that can take some time to heal from so I wouldn't expect you to forget your feelings just to accomodate hers. But that doesn't mean you can't dig deeper with her, and try and empathise, while being honest about how upsetting it is for you.
I guess this is a situation I have been in with parents when I realise that I have to be the one to provide a positive example and be patient while they need to be the ones struggling to learn from me.
Rediscovering and voicing any positives, and demonstrating what a positive relationship could be like between you is bound to help with any defensiveness you recieve.
Like "I'm sorry we argued, are you ok?"
You can ask questions to help you empathise... "Are you scared something bad will happen to me and you will be blamed?" "Are you scared that this is the beginning of me leaving you and forgetting about you?" is it something else?
If she's willing to have those conversations maybe you can offer reassurance or solutions like "I'll rebuke anyone who judges you" or "I'll do everything I can to keep a positive relationship with you as I get older".
I would strongly hope she could appreciate your efforts to understand her, she may still insist, and may not be ready for any of those conversations. I know my family weren't, but I think showing that you have tried demonstrates the maturity you are trying to prove and can make it easier to state boundaries.
You might simply have to continue to do things without her approval, and try not to state those boundaries with anger if you can help it...
Maybe something like: "I'm going to visit my girlfriend even though you've told me not to, I understand that's scary to you and that your beliefs say I should do as I'm told but I want you to know I'm going because I'm growing up and have to start respecting what I believe and care about too, not because I don't respect you and care about you."
"In between two tall mountains there's a place they call lonesome.
Don't see why they call it lonesome.
I'm never lonesome when I go there." Connie Converse - Talkin' Like You