Probably 18 (and I'm 24). Granted, I would have some concerns about her level of maturity for a relationship and discomfort with the fact that I have younger cousins turning 18 who are still like little kids to me. Again, that's the absolute youngest I would possibly date (though highly unlikely), and that number will go up as I get older. When I'm 30+ youngest I think would have to be 21. A 30+ year old has no business "dating" an 18 year old.
Granted, I've never dated somebody outside of a year from me, so a 6-year difference would be pretty crazy.