I contacted OSBI about this very issue a couple of years ago as someone from this forum was going to be visiting OK and was under 21.
At that time, OSBI (who can not give legal advice) believed the individual would not be in violation of any law.
I also believe someone under 21 with an out of state permit would not be in violation of the law. The reciprocity statute says OK will honor any states permit. It makes no mention of an age requirement.
As far as open carry goes, any license from another state will allow you to carry either open or concealed in OK.