That is a tough question and I am sure you will get quite a bit of feedback. Legally once someone turns 18 they can be with anyone they want to be with as long as they are mentally competent. Socially many people may judge this harshly they will say that there is too much of an age gap between the two people, however if both people truly love one another then age really becomes irrelevant. Age is just a number that people go by, the real proof is the maturity level of the people.
There are many things to consider with the relationship. First what are the views on marriage? Are both of you looking at the relationship from a long term standpoint or is this just a fling. If it is just a fling then I would be more suspect of the 30 year olds motives. If you are looking at it from a long term standpoint then you have to think about the things that come with the territory. For instance children. Do either or both of you want children? How will the age difference effect them? Are you okay with each others desires in this area? Next what about the work situation. Who is going to work? Will both of you have careers? Is the 18 year old attending or planning on attending college? What about retirement? One will probably qualify much earlier for retirement than the other is that going to be an issue?
Another issue is your day to day activities? How much do you do together now? Is the older person an active clubber? Does he or she go out to bars and nightclubs frequently, that gets tough when your significant other is 18. Will that become a sore spot? Can you mix friends? And what about dealing with holidays and families? Families can be difficult enough, but if they are very judgmental in this area you have to be prepared to deal with that.
Unfortunately there is no clear answer to your question, because every person is an individual and every couple is unique. Only the two of you can determine your situation and your destiny. I would recommend that you pray about it, confide in your pastor or another reliable Christian confidant you can talk to. Talk to some of your friends and family you are comfortable with. You need to weigh everything and determine is your love true, if it is then true love can carry you through.