I would say that 48x15 (86.4) is a good gear to start on for a 17 year old just starting out on the track.
That gear is actually the limit for racing age 15-16 here in the U.S. In the junior program at our track we use one tooth bigger chainring (above max allowed gear or race gear) to work on leg strength and power. So for our 15-16s they use 49x15 for those workouts. We go smaller for leg speed/spin workouts.
Racing is definitely not limited to one gear. The type of race, the distance, the weather, the track that you are at, even whom you are racing can factor into your gear selection for a race.
My kid is a 15-16, been racing since the age of ten. Has 45, 46, 47, 48 and 49 rings and 15, 16, and 18 cogs. My point is that you will start somewhere and add as needed.
Some might say that a 48x15 is too small. What you need to know is that the same gear you like on a road bike is harder on the track for most people. You also need to develop a really good spin and pedaling form, because when the speed builds, you can't change to a bigger gear. Your legs have to be able to keep up. Better to start out in a gear that is a little small than one that is a little too big.