Is it okay for a 27 year old to date an 18 year old?

As long as both sides are in love and there is no legal issue, no-one should see a problem with it. I would only find it a bit unusual if the 27 yr old was a female and the 18 yr old a male (thinking of typical maturity levels). A former friend of mine (female) once dated a 16-yr-old male when she was 30.