I don't eat as much dirt and worms as I did when I was a kid, I think that might be part of the reason.

If you are wondering "perhaps I am just as sick just as often but I can't feel it" the answer is very likely No, that's not the case.

As others pointed, immune systems are stronger for adults, and children have poorer hygiene habits meaning chances of getting sick skyrocket.

I'm struggling to find the charts but yes, frequency of illnesses caused by bacteria and viruses is high in children and seniors and low in adults (unless the adult is often around a child or senior). It's because the immune system is strongest in adults and weakest in the very young and very old.

If you don't hang around kids or seniors, you'll rarely ever get sick.

If, as an adult, you have a kid and send 'em to daycare...you're probably going to be getting sick "pretty regularly" again! (It's worth it, but it does suck.)

I don't get sick as often, but my head hurts and I feel like trash much more often.

I don't understand what you mean about feelings. But we definitely are in much deeper contact with other humans. When I was younger I lived on the countryside, with very few people to exchange illness with. It's the complete opposite today, and I'm still sick way less often. So I would say my immune system improved

Possibly because you go out less, around a smaller set of people and/or aren't doing as many gross / close things as younger people.

Yes.