We have to define decent. Any way we choose will be arbitrary, but we have to start somewhere. Let's assume mile times follow a normal distribution and use half standard deviations to define our groups. I've applied the qualitative descriptions.
Below average - less than 50th percentile
Decent - 50th to 69th percentile
Good - 69th to 84th percentile
Very Good - 84th to 93rd percentile
Outstanding - greater than 93rd percentile
Also we have to define our population. Are we talking about all male American sophomores, all male American sophomore track athletes, or all male American sophomore distance runners. It's an important distinction. At my school, for example, there are 2 sophomore boys who can run a sub-5 mile. Out of the 1000-person school that's 2 out of 125 sophomore boys. But on the 50-person boys track team that's 2 out of 12.5 sophomore boys. But on the 16-person distance squad, that's 2 out of 4 sophomore boys.
So within the context of the entire school they are outstanding, within the context of the track team they are very good, and within the context of the distance squad they are decent. So context matters too.
We tend to compare ourselves within our event, so I think the strictest criteria applies. Take any 10 random distance runner sophomore boys. Good, bad, great, terrible, whatever. The criteria is that they run distance competitively and you choose them randomly.
Be honest with yourself now. You should have a sense of where you stand. Are you pretty confident you could beat 5 of them? If so, congratulations, you're decent!