I was 7 weeks old when I flew NYC-SXM. I was 5 months when I flew JFK-TLV. It's more about how you're handling things than how the child is. You'll learn your child but be ready to deal with situation as they arise.
Don't expect people to be nicer in situations just because you are traveling with an infant. People will often nicely offer to assist if you're traveling alone to give you time to run to the restroom, eat or even just walk around for a few. I normally offer if someone in my aisle has a child unless the thing is a hellspawn.