From healthcare.gov:
How premiums are set
Under the health care law, insurance companies can account for only 5
things when setting premiums.
Age: Premiums can be up to 3 times higher for older people than for
younger ones.
Location: Where you live has a big effect on your
premiums. Differences in competition, state and local rules, and cost
of living account for this.
Tobacco use: Insurers can charge tobacco
users up to 50% more than those who don’t use tobacco.
Individual vs.
family enrollment: Insurers can charge more for a plan that also
covers a spouse and/or dependents.
Plan category: There are five plan
categories – Bronze, Silver, Gold, Platinum, and catastrophic. The
categories are based on how you and the plan share costs. Bronze plans
usually have lower monthly premiums and higher out-of-pocket costs
when you get care. Platinum plans usually have the highest premiums
and lowest out-of-pocket costs.
Factors that can’t affect premiums
Insurance companies can’t charge women and men different prices for
the same plan.
They also can’t take your current health or medical
history into account. All health plans must cover treatment for
pre-existing conditions from the day coverage starts.
https://www.healthcare.gov/how-plans-set-your-premiums/