Both my sons - one born in July, now in 4th grade and youngest born in August, now in 2nd grade - entered K at age 5. They've not had a problem being the "youngest," since there are also other kids in their grade born in July or late August (they're definitely not the only ones)
In my older son's class there's always been one boy who started K at age 6 - he's always seemed much older (not just than my son, but than the other kids as well); his parents say that waiting one year was the right thing to do in his case, so it's really up to you to decide, each situation is different.