A cause of COVID-19 is pneumonia. Pneumonia can affect people of any age, but it's more common, and can be more serious, in certain groups of people, such as the very young or the elderly.
Most old people are staying in their homes and following advice. The young thing they're invisible so are continuing to do as they wish or putting themselves at risk to help the more vulnerable.
This may be why the stats are indicating there are more deaths of people younger than 70, but that's not to say i've seen the stats. I haven't.