Some things I've noticed in my early 40s:

• You talk about things like property taxes and 401k contributions more often than you ever thought would be possible
• You seriously weigh whether a drink is worth the bad sleep and headache it'll cause you
• Your pop-culture references are lost on younger folks
• You start referring to college-aged people as "kids"
• You need reading glasses but you're in denial about it
• Injuries take longer to heal
• Those of your peers who haven't taken care of themselves are starting to have serious health issues
• You care more about flossing, skincare, fiber, and hydration
• You still rock bottom eyeliner like you did in high school
• You've seen fashion from your high school years go out of style and then come back as "retro"
• You see the utter confusion on your nephew's face when you explain that you used to keep a quarter in your bike pouch in case you needed to call someone, because he doesn't remember a time before cell phones and his generation isn't allowed to just roam around unsupervised on their bikes
• You have strong opinions about things like laundry detergent brands
• Birds become fascinating
• You have no problem spending a few hundred dollars on a new kayak paddle, but the price of cold cuts these days is just unacceptable