Out of the mild cases, some are comparable to severe flu with symptoms like cough, fever, achiness all over and sometimes (10%?) diarrhea. There's a difference in that symptoms common in flu and colds such as runny nose and sneezing are not associated with Covid-19, this is more targeting the lungs. Something like half of mild cases would get pneumonia i.e. bed-ridden with fluid in your lungs and difficulty of breathing; that's significantly beyond most cases in flu, but comparable to the complications of a bad flu, which sometimes results in pneumonia. That's counted under the mild cases, as there are no invasive procedures required and you're going to survive this without hospitalization - that is the criteria for a mild case, it doesn't imply that the patient would call it mild, it means that they'll feel shitty in their bed but not die.
Moderate cases (perhaps 10-15% of total, reports vary significantly between countries) have severe pneumonia that would require hospitalization and supplementary oxygen. If you try to go to work with a case that turns out to be moderate, that's where you can become the "videos of people collapsing" event because during that single day you can get worse and start lacking oxygen. Still, you're going to be mostly conscious and survive that if you're in a hospital, i.e. if the hospital system isn't totally overwhelmed.
In severe cases (something like 5%) you can't breathe on your own even with extra oxygen. This requires intubation and ventilation for a prolonged time (weeks), which is a limited capacity (both in terms of ventilators and of the skilled medics needed to do that), a big strain on your body, and you're going to die if hospitals can't provide that. And there are reports from Italy that even right now with just 7500 confirmed infected cases they can't ventilate everyone who needs it, and they need to triage who won't be getting it.
We have data that older people are much more likely to die. However, as far as I know, data does not suggest that young people are less likely to get hospitalized and need invasive interventions - it's just that they are strong enough to survive the ordeal. The case of Italy/Lombardy first known case may be illustrative of severe cases in younger patients - it's a 38 year old marathon runner, he's supposedly released from intensive care yesterday as he's able to breathe on his own again. He was hospitalized for a bit more than three weeks, two of those in intensive care with artificial breathing/ventilation.
Most likely they are referencing this study showing 40% of people who had SARS had long lasting effects https://jamanetwork.com/journals/jamainternalmedicine/fullar...
While COVID-19 mutated from SARS it has a number of characteristics that are different so I don't think it's reasonably to assume it'd be the same. Could be worst or better.
Many survivors end up with "honeycomb" patterns in their lungs because the immune system response blew holes in alveoli, which then merged together as they healed.
A recent study on the novel coronavirus found that 14% of patients ended up with this honeycomb pattern as well. (They call it "ground glass opacity".) https://www.thelancet.com/journals/lancet/article/PIIS0140-6...