It's not that they are weaker, but that many things actually confer long term immunity. The "common cold" is actually many viruses, once you've beaten one it's not likely to get you again but it's cousins will.

Not as much as their immune systems haven't been around very long and seen as many diseases.

I was under the impression that younger kids had stronger immune systems for that reason, which is why the common wisdom is to expose your kid to as much as possible when they're young, because it's easier for them to fight it off and later rely on resistance?

I guess it's a quibble over the meaning of 'stronger', but that seems overly pedantic. Let's get data.

A good overview of the allergy situation is here: https://en.wikipedia.org/wiki/Hygiene_hypothesis

Though that article goes into more about young human immune systems too. TLDR: Yeah ... it's complicated, but the evidence seems to, just maybe, lean towards more exposure being better. So, you're right, I think?

the impression I got from the article is that it's a somewhat unsolved problem. the article mentions an "old friends" hypothesis that suggests that it's not exposure to dangerous pathogens that's important in childhood, but exposure to benevolent ones. but yeah, you're right, it's semantics really. kids have a different kind of strength. I would actually suggest it's quite analogous to brain development. kids are more adaptive and I'd argue creative, but adults are more experienced.