> What I'm curious is about is the percentage of people (younger or otherwise) who either develop no symptoms or develop symptoms so mild as to not be attributed to the virus. Do such people exist?
Yes, absolutely. Many countries such as England do community infection studies. Essentially they pick a random sample of people and test them every week to see many people have the disease, even if they don't know it.
In the England study, they report that ~70% of positive tests involve people who didn't report any symptoms ever. That implies there are lots of people who had it who didn't know they had it.
> Out of those people that tested positive for COVID-19 over the study period, only 23% (95% confidence interval: 15% to 32%) reported experiencing one or more of the various symptoms at the time of their test. Out of those who reported testing positive, 33% (95% confidence interval: 23% to 44%) reported experiencing symptoms at any point in the period around testing positive. This was at the time of the visit, or at either the preceding or following visits.
https://www.ons.gov.uk/peoplepopulationandcommunity/healthan...
Of course, the question is do these people get counted later in other ways - i.e. what percentage of them would show up in antibody surveys? That is currently a difficult question to answer with a high level of certainty for several reasons. Right now, we don't know for sure what level of infection causes you to develop detectable immunoglobulins, we don't know how long those stick around, and we don't know how much our immune system leans on T cells instead to fight COVID which don't show up in the existing antibody tests at all and are much more difficult to test for.
And it's also worth pointing out that it's not clear that asymptomatic infections are as infectious as symptomatic infections. So don't assume they are equal. A recent study showed that asymptomatic people could shed the virus for 3 weeks, but it's really hard to know if that's active virus that could infect someone or not. There's a lot more research to be done.
See https://blogs.sciencemag.org/pipeline/archives/2020/06/22/th...