I know you may have heard this before (so I will try and only say it this once), but everyone’s body is different. This means that not only are people’s bodies shaped differently, but they mature at different rates and in general have different things happen at different times.
That being said, your body is probably just going through the “wonder” that is puberty in a different way than your friends. It is entirely possible that many people around your age who do not have flat nipples still did not get their period… basically, no two bodies are going to go through the exact same things in the exact same order at the exact same time.
Your nipples may become less “flat” with time (as fat and muscle tissue redistributes in your body, etc.). They may also remain “flatter” than same people’s and that’s ok too. Keep in mind that not only do everyone’s nipples look different but they respond differently too.
In general nipples tend to be more flush with the rest of the breast and softer until they are reacting to some kind of stimuli (being turned on, excited, cold…). Some people’s nipples get pointy and hard at the slightest change in temperature!
Also, breasts change over time (especially when you’re a teen) – they grow and shrink with other changes in your body, and one may even get a tiny bit bigger or smaller than the other, etc. You also mentioned that you are “athletic” which can also impact the way the fatty tissue that makes up your breasts gets distributed (playing sports/being athletic builds muscles and generally burns fat, so that can impact the shape and size of your breasts).
A cgood website you might want to check out is: http://www.scarleteen.com
Thanks so much for your question! I hope this answer helped. Feel free to write back if you have more/other questions.
-s, for AlterHeroes