What can I expect within a year?

It really depends on your training. I think nobody can give a definitive answer, especially since we don’t know your starting point (your average speed doesn’t tell much without knowing the duration, track, bicycle, conditions, effort etc. etc. etc.).

Since you seem to be focused on speed: Speed mostly depends on your power output versus weight (yours+bicycle) and aerodynamic drag.

This table gives an overview on how much power cyclists of various categories should be able to output (in watts per kg of body weight). To compare bicycle performance it’s really best and easiest to use power or power per unit of mass. If you want to track your training and progress you should consider getting a power meter.

You can play around with this calculator if you want to see how power affects speed:

http://www.kreuzotter.de/english/espeed.htm

The “bad” news for you is that it’s a game of diminishing returns. Using the default values for a road bike you can see that increasing power by a reasonable amount from 148W for 16.8mph (27km/h) to 180W only increases speed to 18mph (29km/h).

This is really an unanswerable question. You might have only moderate potential or be the next Peter Sagan. We also know nothing about the area or environment you ride in - that ~17pmh/27kph in dead flat roads is not as good as on roads with a lot of climbing. Also, how long are you holding that average speed for?

All you can really do is focus on getting better. Start using a ride tracker such as Strava so you can track progress. Decide what type of riding you want to do: short fast rides, serious climbing or long distances? Research cycling training for what you want to do, make a training plan and execute it.

You're still young - at 15 you are still growing, so work on endurance and technique, and not too much on raw power.

And remember its not about ultimate top speed, there's got to be an element of enjoyment too. So work on enjoying your time on the bike, but also have a life off the bike.