Is it bad that I dream about kissing a girl that is not my gf? A girl who is too young for me?

No, dreaming (or the subjects/stories therein) is never bad.

Dreaming is fine. Touching under-age girls will get you into lots of trouble.

It is ok as long as it is a dream and not actioned.

Alright, well, how do you feel about this girl in real life? If you think you feel attraction towards her, you should consider figuring that out internally. It would really hurt your girlfriend if she found out you had feelings for someone else as well. I’d also like to emphasize I’m really proud of you for putting yourself out there!! Dealing with these topics are difficult, so I commend you :) But yea… try and take control of your thoughts! Whenever I’ve found myself interested in another person while I was in a relationship, I tried to reverse the roles. How would I feel if my S.O. was with someone else?
Things you can do to take action against these thoughts, are stopping yourself whenever you find yourself thinking of that girl. Try to think of one reason why you like/love your girlfriend instead. Try to remind yourself of fun memories with her, or go on dates more often!
All the best,
itsnuggettime

@itsnuggettime I disagree with all the action, internalizing and thinking about this. It is a dream and insignificant to apply so much thought over.

It’s very normal, don’t worry about it.

If it is a dream your fine. If it is real life your not fine.

You are all your dream people.

Nope dreaming is fine. Touching in life is bad. But if it’s a matter of feeling guilt, by chance were you younger in your dream? Like when I dream of my husband or anyone else younger we are younger than I am now. I’m not old and he’s young. I’m not sure if my mind is telling me love is only for the young or it’s my mind’s way of not creeping me out. I feel it’s in my mind trying to not creep me out. Actually thinking about it, most of my dreams have me being way younger like in my 20s if sexual and as a child when dreaming of my family.