It's do with living in the moment vs spending your time thinking about what you did or worrying about what the future might bring, IMHO.

We become way more prone to spend our time doing things like thinking about stuff we did (and how we miss it if it was good or could've done it better if it was bad) and worrying about what the future can bring (and not necessarily in grand terms: somethingas simple as "I have to get a haircut" which then goes one to "when will I have the time", then "but I need that time for X" and so on) as we grow older.

You absolutelly can still have some moments of wonder (for things as simple as how a cobweb looks with droplets of morning mist on it) but you need to be present there in mind also, not just in body, and not to not let some memory or concern rush in to take your mental attention away from the now.

I had a point in my life with a ton of anxiety and ended up learning Mindfulness (which is simply to try and not say anything to yourself in your mind, which is surprisingly hard to do for more than a few seconds) to stop the feeling (if you're not constantly looking back to something bad or fearing for something bad in the future you don't feel anxious about those things) and as a side effect I ended up with the habit of being more often present in the moment and that's how you just enjoy little wonders when you come across them.

Still, it's nowhere at the level one has as a kid.