Younger people do sometimes have underlying conditions, and as
@kristiang85 says most will know it. And I don't want to belittle yours or others people's potential conditions, but it really is up to those individuals, their medical professionals, family etc to understand and mitigate for any risk.
Nobody can expect the rest of society to curtail their lives for the extraordinarily small risk that they might come into contact with, and infect someone at greater risk. Its just not feasible.
There will of course be some people unaware of their risk, and speaking as someone who lost his sister to just such a situation following her having had flu I would still not expect anyone to do what we have been asked to do for the last 14 months.