You have to think that the affirmative sentence is 我是18岁and to make the negative form you have always to add 不 before the verb.
It's also true that you can omit the verb 是 in the affirmative form and the sentence still works. So 我18岁 it's OK, it appears maybe more colloquial. But I've never came across a negative sentence like the one you proposed.
This is my conclusion, but I suggest you to wait for a native speaker to be surer:
我18岁 = I'm 18
我不是18岁 = I am not 18 years old
我不18岁 = I not 18 year old
我是18岁 is grammatically correct and would appear in certain context:
But 我18岁 and 我今年18岁 and 我十八（我今年十八） are more natural. I think most people would omit 是.
To negate a sentence in Chinese is not so easy as in English. The negative form of 我十八 could be, directly, 我不是十八岁. And 我不是十八岁 is correct but less common than 我没有十八/我不到十八/我没到十八, though the latter three mean " I'm younger than 18".
I hope you don't mind if I ask a question about this sentence. I've heard 又 used in this type of context before but am not 100% sure of what it means. Would anyone be able to explain its meaning?