Er...are they bringing up the age difference just because? Or are they bringing it up in relationship to some issue you are having?
I wouldn't recommend anybody age 18 to have a kid period, and if they wanted to I'd suggest to do so sanely would mean having at least a two year long DATING relationship with the person you are seeing first with a period of cohabitation (no I don't think just knowing them longer is enough) to determine if this is something that can really work long hall. I feel the same about marriage. It is easy to forget to be practical and sensible when you're feeling all romantic. I'll say from experience, most people end up regretting being hasty.
Happiness will never come to those who fail to appreciate what they already have.