I flew with my son at 4 months. I gate checked the stroller to go under the plane. I have never seen anyone allow a stroller to go on the plane. I think it's probably too big for the overhead bins. If you buy an extra ticket for your baby you should have no problem taking it on board with you. If you change your mind on the baby using it on the plane I'd HIGHLY recommend you purchase a car seat travel bag, I've even added extra items inside it with the car seat and it checks for free with the rest of your luggage.
I flew with my son at 4 months. I gate checked the stroller to go under the plane. I have never seen anyone allow a stroller to go on