It's just saying that 7% of the total people they surveyed fall into the group "men 18-29". Without knowing how many people were actually polled and how they were selected we cannot say anything about the statistical validity of the sample.
According to wikipedia, 15-24 age group is about 13% of the population. 18-29 is a slightly older and wider range, so could be 14%. Then obviously only half of those are men, so we end up at 7%. That's totally plausible