I think it all boils down to your rules. I don't really think you should change the age limit because you have a valuable 14-year-old on your site. When I was 12, I got kicked off of a site because the age-limit was 13. Nobody knew I was 12, nobody really cared—until I casually mentioned I was 12 (I was unaware of the rule). Rules are rules, so I was kicked off the site. It's part of the administrator's job (your job) to make sure the rules are enforced, and if you make exceptions for this one member, there is nothing to say more members won't join your site under-aged or break other rules because they think they aren't taken as seriously.
At that point, you may as well lower the age-limit since you aren't really enforcing the rule. So, the idea would be to go to the reason why you have the rule. What you may want to do is email his parents (if possible) and let them know of the situation. They will most likely get him to leave the site. If they don't, you might just have to tell him "sorry, but you can't be part of this site." He has violated your terms of service, and unfortunately, he must wait a couple years before joining the site again. You can try giving him some links to sites like yours, making it clear that you think he is a good guy, but he needs a better environment to be in and you are concerned that an 18+ website is not suited for him. Despite the fact that's not why you have the rule, it may be there anyway. Because it sounds to me that he is (unintentionally) being disruptive. Members are emailing you. And again, if you show laxness with your rules, people will be less likely to take them seriously.
One more thing: You definitely don't want to get in trouble with his parents. I know that it won't be your fault, but the parents will find out he is on this site sooner or later, and if/when they do, they might see things they don't think are appropriate. I can't say I've seen your website, but even so, I doubt his parents will have much of an open mind if they do, in fact, go on the site themselves. That, added to the fact that you did not enforce the rule will probably make them rather angry, possibly making a public forum post complaining about it and thus, deepening the hole. I'm not saying this is likely to happen. In fact, I don't really know what sorts of things are on your site or how concerned the parents are about his online safety, but it may be worth not taking the risk.
Bottom line, there are rules against what he is doing, and you are the owner. It's up to you what you do, but even if he is an exceptionally bright member, you may have to take action accordingly. He'll probably understand that you're just doing your job. And it's not like he's the only valuable member there. Just recognize that there are other things at stake here, such as the reputation of your site.
Good luck with whatever decision you choose, though. In the end, it's all up to you.