Nothing wrong with saying stuff like that.

I don't see a problem with being a bit immature. I'm a 27 year old man but more often than not I act like a teenager. I don't see the point of acting "all grown up" if it reduces my overall happiness.

So yeah, if I had a girlfriend I would say stuff like "love you to the moon and back" or if she said "I love you" I would reply "I love you more". That's just how I am and how I will remain 10, 20, and even 50 years from now.