There is more to a premium than its definition of it representing an amount an insurance company charges for a particular policy upon purchase. Premiums may be raised under particular policy provisions, such as level premiums, attained age and issue age. Level premiums go up upon the raise of premiums on all of the same-type policies as set by the insurance company under the allowance of the state insurance department or commission.
The market force and state regulation will both keep this attribute under fair maintenance. Attained age is contained in policies which base premiums solely on age, which will issue the same amount to everyone under the same policy with the same age. As one ages, the premium readjusts to the price set to those of the attained age, in variable increments.
Under a policy in regard to issued age, your Medigap policy premium will be set at the same amount as anybody else who were to have purchased the policy at the same age. Under such a policy, the premium will not necessarily rise retroactively in accordance upon reaching age marks (for example 70, 75, 80) as it would under a policy which is issued in regard to attained age.