If you’re under 400% FPL in SoCal, age doesn’t matter much, ACA tax credits, drive your cost for coverage to a common point.
San Diego has an anomaly, in that the HealthNet plans for older people actually have a premium that is below the ACA credit level which is determined by taking (I think) the average of the three lowest plans and calculating a max percentage of income to purchase health coverage.
In other words, whether you are 20, 40 or 60, at a certain income level, ACA credits push your net cost for coverage to ~4% (at $18K single) range and I think 10% at 400% FPL. It doesn’t matter how much the average premium costs, the tax credits push it to same level.