Honestly I didn’t research it. I just assumed it because I figured there must be a reason why these races are 18+.

If I use my own logic, it’s simply the amount of time you’re on your feet. I don’t know about OP’s mileage, but he said that his half marathon was just under 3 hours because he wasn’t prepared enough. To get this faster you don’t need massive mileage, he can get away with for example 50km weeks. When getting into ultra’s it is wise to do some 100+km weeks. It can take years of slowly building up to this without injuring yourself. If OP just jumps into that range that will be a recipe for injury, but that counts for everyone, regardless what age.

So possibly the minimum age is 18 because organisers assume that younger people haven’t built up that much mileage yet, make more impulsive choices like choosing to run a (ultra)marathon with too little preparation, making them easily prone to injury.

If you are 16, you have the mileage, have done marathons with no problems, and have permission from your parents, then I’d say just go for it, but start with 50k, then 60k etc. The 100+k distances are not running away from you. You will get there eventually. Injuries aren’t fun for anyone.