Most Helpful Guy
For a teen it is certainly a bit too big, due to even more important factors, namely maturity and different life stages. At 18 he is in a transition from teen to adult, learning to take the work and responsibilities a grown-up has to do, like earning a living, getting a job or into college, and surviving in the society, while at 15 life does not get far from school and other respective cliques and groups. The mentality and daily schedules are a lot different between you two. It will be hard for you to cope with each others' lives.
The age range widens at adulthood, but for now it safer to go for someone no more than a year's age difference since than minimizes the differences I mentioned.
Most Helpful Girl
Not really considering it's only 3 years... But at the same time it is because you're young. You're either a Freshmen or a Sophomore and he's a Senior.
It's only going to comoliment things. He could be using you , hopefully he's more mature than you, and then not to mention he's going to be leaving you and off to college.
So in the end it's up to you and your parents but it's better to not bite off more than you can chew. It won't make a difference in a couple of years but while going through puberty/maturing it will.