Well, your question is very circumstantial--legally, yes you are "underage", because as an 18 year old, he is a legal adult, and as a 15 year old, you're still a minor. So, basically, yes, you are underage for this guy until you turn 18, yourself.
Aside from the legal issues presented--there are some moral issues. Are you at the same level of maturity to be in a relationship with this guy? If you can't honestly say that you are, you're probably too young for him. In most cases, 15 year olds and 18 year olds are a little too large of a teenage gap for you to honestly relate and maintain a healthy relationship--however, there are always some exceptions.
Basically, you just must ask yourself if you're too young for him or not.
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