It could also be said that by being too heavy handed you let the terrorists win, or create more than you remove. I get what you're trying to do but there's a reason porn sites don't have the "are you 18" checks anymore. I'm sorry dude, this all seems a bit silly.
No dude, its redonk to ask in the first place. Whether or not people respect your wishes is irrelevant as this shouldn't be an issue in the first place. I cannot for the life of me fathom why you would need to be 18 years old to read a guitar forum. Are you trying to keep out the rabble? The people who will act like children are the same people who won't adhere to some arbitrary age restriction. I can also guarantee you, after being here for over ten years, that your soon to be worst offenders are all over 18!
When you delete this post remember that criticism is important. Your ultimate success depends on us so its in your best interest to listen.