My thought is, it is what it is. Is it immature? Sure, probably. But she's basically still a kid. She's not "acting" immature. She is immature. She's 20. Last year, she was a teenager. If you want to date someone who behaves like a 44 year old, then date a 44 year old. If you want to date a 20 year old, then be prepared for the Millennial Generation baggage that comes with it.
Found this quote on another thread here. All I could do was smile and think YUP!
I suppose it's hard for some people to accept, but there IS a difference. Our life experiences create who we are to a large degree and the more life experiences we have the wider our range of comprehension and knowledge about how different behaviors affect ourselves AND OTHERS.
There is a reason I don't date people so much younger than me. Its not because I can't adore them. I have a LOT of early 20's people who I care deeply for. But in the scheme of our life experience, they are still "younguns".