In principle you could isolate just the higher-risk individuals, but the age limit you'd want to avoid overwhelming the healthcare system is wayyyy lower than 70, probably around age 40 where the CFR is maybe 0.2% [1]
You could — if you thought that would slow things done enough to prevent overwhelming the healthcare system. At the moment it looks like we probably haven’t done enough, seems crazy to consider doing less.
It’s not like younger people are a self-contained sub-population. Increased rate of transmission among younger people means an increased rate of transmission to older people too.
1. https://www.worldometers.info/coronavirus/coronavirus-age-se...