We flew when both our boys were 4/5 months old (obviously different years). No issues at all and they were long haul flights.
However, I once flew alone with my eldest when he was 2 years old and I still suffer from PTSD. It was a six hour flight and we had super economy seats - yes, more leg room but it meant I couldn’t block him in. He kept wanting to go to the toilet, wanted drinks, dropped food, ran up and down the aisle, refused to sit on my lap, refused to sleep, couldn’t be distracted by anything, no Pingu, no games, no colouring (This was before iPads were commonplace). Passengers kept giving me dirty looks, I spent 45 mins in the toilet letting him play with the taps as I cried over my failures as a mother. He promptly fell asleep at immigration.
I don’t want to put you off tho! This never ever happened again but my advice is to plan well. Tire them out as much as possible beforehand, bring treats but not sweets (I doped my son with haribos so he was wired on sugar!), fly economy and sit them by the window so they can’t get out!