What a terrible agest remark! Maturaty is most certainly not an age thing.
Sure, more often than not, twelve year olds are going to "strt tlkin lik dis". But just because your eighteen, it doesn't make you auto-mature I'll have you know. Besides, your getting rid of a whole section of people that would help boost your forums popularity.
The moderators should be the mature ones, you can't just choose a mature audience, there is no stereotypical age that's mature - besides people that can't use computers very well
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Besides, kids can lie, so you may aswell set it to somewhere lower so they don't get annoyed at the age restriction.
Anyway, design? Well the colours need to go with eachother, three main colours usually do the trick. Personally, Blues, whites and greys are often used (Not neccessarily together) and generally look nice with alot of colours.
Content? Well it all depends what your forum is about really.
Back to the mods, firm, friendly, but fair does the trick.