Most boxers seem to look at it as a hard-knocks contact sport for the young. To a certain degree, this is true. Once you pass your prime you really can't "mix it up" with the young bucks like you used to. Age robs you of speed and stamina. Yeah, a 55 year old boxer who keeps "in training" will clean the clock of your average bozo off the street. But, as a general rule, he's not going to be able to keep up with a 25 year old boxer in daily training.
Those who truly love it, and there are some, will keep sparring but they stop trying to box at a competitive level, which is, honestly, bread and butter for boxing. There's a reason that Foreman is selling grills now instead of pulling title cards. I wouldn't want to go a few rounds with him, but he's not going to be defending any professional belts.
Peace favor your sword,
Kirk