Physically, I think you start to notice that you aren’t 20 anymore in a way that you don’t notice in your 30s. Like playing sports or doing different things. My husband notices it too. If you have 20/20 vision like I do, eventually you will start to need glasses for reading-- which I did this year (turning 43 in a few months). I likely have gray hair, but I color it so I don’t really know how much is there now!

As a woman, I’m starting to notice little wonky things in my cycle-- changes that are probably telling me menopause is coming. Yikes!
But, physical slowing down aside-- 40s are pretty great. I have found myself having much more of an "I don’t give a cr
p" attitude. As in I don’t give a crp what other people think, whether my house is perfectly clean, whether I put on makeup to go to the grocery store… etc. I’m sort of at a place where “this is me, love it or lump it.”

I think that comes with confidence and experience in who you are, and your worth (as a child of God, but also in your profession or what-have-you).
I am more outspoken-- not caring if someone disagrees or what they “think”-- which I think when we are younger means much more. I’ve set higher expectations for myself and others around me now than when I was younger. I’ve deepended my faith a lot since my 20s, and after sort of a dip in my 30s due to being busy with grad school and life, now I find myself getting much more introspective and picking up some of that zeal regarding the faith that I had in my 20s.
I think personally I’ve gotten a lot more introverted too. I want more time to myself. More time to reflect and read and enjoy life without going 100 mph.
I am sure everybody is different, but that’s my experience as a 40-something. I think age is just a number, it’s what you do with it that counts!