What's actually causing it
Small nerves run between the long bones of your forefoot to supply your toes. A Morton's neuroma is a thickening of the tissue around one of them, after that nerve has been irritated or damaged 1. The change is scarring and wear around the nerve, which is why some clinicians say "interdigital neuritis" (an irritated nerve between the toes) describes it better than "neuroma" 6. You're not dealing with a lump that needs removing. You're dealing with a nerve that's being squeezed.
The squeeze happens where the nerve runs under the ligament that crosses your forefoot, most often between the third and fourth toes 6. One proposed reason: the nerve there is thicker and less mobile than its neighbours, so it has less room to move out of the way 6.
What loads it up is consistent: tight, pointy or high-heeled shoes; running, jumping or other sport that drives weight through the front of the foot; and other foot problems, including flat feet, high arches, bunions and hammer toes 1.
Around here, it's often work rather than sport. A safety boot with a narrow toe box, worn through a full shift on hard floors, squeezes the front of the foot for eight or ten hours at a time.
It's around five times more common in women than men, with an average age around 50 at the time of surgery 6. Being well under that doesn't rule it out.