The OP seems to believe his chance of success reduces if he waits a year. He can get his teeth fixed post training anyhow.
Based on what? All the stats imply that slightly older candidates tend to do better at the stuff that matters, i.e. IOT and EFT and beyond. He'll arguably increase his chances by waiting. We've all seen plenty of keen youngsters get chopped, and not having a degree/getting your teeth fixed/etc might be a big regret if he doesn't pass training.
I think a realistic attitude is better than all the "reach for the stars! Follow your dreams! The sky's the limit!" stuff bandied around. If he passed the aptitude tests well, he'll almost certainly pass them next year. With an extra year's maturity and growing up behind him, he might do better at the leadership selection and interview stages. An extra year's prep won't hurt!
They might recruit less next year. On the other hand, they might recruit more, meaning he'll have an even better chance!
If he was bouncing off the age limit and about to turn 26, then yes, take the risk. But at 18?