Age in a relationship..how old is too old?

Is it okay for an 18 to date a 27 year old? The 18 year old is turning 19 in a month & the 27 year old just turned 27. So therefore there is only a 8 year age difference. When she's 20, he'll be 28. So should age matter in a relationship? If the two parties involved in the relationship share equal beliefs in sex, religion & so forth, should an age gap be an issue? Not an issue between the two people but an issue for other people I guess. Lately I've been hearing mixed opinions. Some say a 27 year old dating an 18 year old is wrong. However the 18 year old just met the 27 this month & as stated before, will be 19 in the next month. So what's wrong with an 8 year age gap?