Oklahoma law is very clear on this. The amount of coverage is reduced to what the premium would have purchased at the correct age. (So you'll have less coverage). This could actually be worse - I'm sure an insurance company has argued that the policy should be null and void if the insured lied (or was mistaken) about his/her age on the application.
The relevant statute is:
36 Okla.Stat. ยง 4006.