Compare this to a individuals wanting health insurance - if they're young and healthy they're less likely to want to pay much for insurance, which leaves older and sicker people to pay higher and higher premiums.
That's NOT what adverse selection is. That's simply actuarial pricing - everyone pays for their own risk.
Adverse selection is caused by information disadvantage - the party buying insurance has information the insurance company does not have or is unable to incorporate into the price. https://en.wikipedia.org/wiki/Adverse_selection
The main cause for adverse selection in health insurance markets are regulations against pricing in various forms of pre-existing conditions.