Apparently you were way old when you grad HS, if you did grad. Not a math major either.
Frank is 17 now and will be 18 when he graduates with his class. Although he may graduate early. Average age for HS seniors is 17-18. Look it up and do the math, if you can.
I was 16 when I graduated and that's not all that unusual. Two years younger than Frank will be. I know, you can't figure the math out on that one either.
You're also wrong about Frank possibly enrolling at Stanford while he is still 17. No, that won't happen. Apparently you don't know why not. Look it up.
So, IF, and it is a big IF, Jackson does go to Stanford, he will be 18 when he gets there. Same age as most Stanford FR and certainly NOT young for the class. However, if he sticks with his plan and goes on a mission then he will likely be 20 when he gets to college, wherever that may be.
This LDS singles discussion is really stupid anyway since Frank will be too busy with other things to care about dating anyway. In any case, it won't be a problem for him. He'll be just fine, despite what you think.