Are me and my girlfriend too old to be acting like this?

No way man, you are never too old to do anything as long as you enjoy it. I'm 21 and I still do childish stuff with my gf. I lose track of how many times a day I will say something stupid to her or ask her to do things like build a fort with me. I wouldn't call it wild, the whole sex in the field thing is kinda hot too, so if you enjoy what you do more power to you man.