i flew to texas (a 3 hr flight) when avery was 9 weeks and it all went really smoothly. i fed her on take off and landing and she slept the entire flight both ways. i really think it was harder on me to have to be stuck in a teeny tiny spot with an infant who at any moment may wake up and start howling. i was seriously petrified that i would have "that baby" that cries the entire flight- luckily she didnt.
also i had to bring a TON of stuff because my mom had nothing- so the carseat AND base, stroller, wipes, daipers, etc- i mean it seriously was a ton of crap.
i really didnt enjoy the whole traveling with a baby experience but like tammy said i am sure it was easiest at this age when they will sleep and you can hold them the whole time compared to when they are moving around more. and i just sort of felt bad for avery because the entire process of flying takes like 5-6 hours for a 3 hour flight and it just seems like a long time for a baby.
bottom line: if you go make sure you sit somewhere with a little more space, bring lots of bottles or breastfeed, bring toys, and last case scenario you can give them a small amount of tylenol to make them a little sleepier ( i didnt have to do it but was prepared in case i had to)
also think about once you arrive- will being there with a young baby be fun for you? how much help will you have? think about the climate- how hot will it be? will you have to be sitting inside with the baby while everyone is out site-seeing? i know personally my trip to texas was exhausting. i was staying at my moms so she helped alot but matt wasnt there to help. so it was a little rough because when everyone was outside we would be inside (babies younger than 6 months arent supposed to be outside for very long) and then i had to feed her every 3 hours and hold her and put her to sleep and bathe her- and she was totally overstimulated and out of her normal routine i was exhausted by the time we got home. but she was much younger than your child would be- ask tammy how jack did in mexico since he will be closer to the age of your kid