I took my son to his first game at 6 months old. I have season tickets, on the aisle so being able to run in and out with him wasn't an issue.. If you are going to do it I highly suggest aisle seats. I also took him to a sunday afternoon game, those are rare and most family friendly.
He did fine, the game was on superbowl sunday so we didn't stay the entire time, i'm not sure if he would have made it through the entire game. While there he was in awe by the lights and fast movements on the court. We also purchased baby earmuffs that he wore the entire time.
I wouldn't have changed the experience for the world tbh. It was pierce's last game at the garden (with the clippers) so i wasn't there for his final three at the buzzer. That said even if I hadn't gone with my son I doubt I would have stayed for that moment due to it being super bowl sunday.
I also took him to a game this season, at 2.5 years old. This experience was a lot worse.. He hated it, didn't want to sit still, was overwhelmed by the crowd and noise and only wanted to really run around the hallway. I think it'll be a few more years before he comes back with me.