His height does mean he can put on a large amount of muscle which as stated will help in his upcoming events. That is not to say being shorter is a huge disadvantage when you factor in the different points that cause someone to be physically stronger than the "average" person.
Good genetics come into play - a strong father and mother means that the child will have the ability to become quite strong, the points where tendon connects to bone and thus greater leverage, the length of the muscles, obviously the size of the person and one factor that is not always included (it is not a physical attribute) - mental mind set (the determination to be the best).
Now if we assume that many people are gifted enough to have one or two of these factors (even if you do not, the potential to become strong is still there - what I am getting at is it may well take more time to develop this strength) it means they can build large amounts of strength.
For those who have good genetics, good tendon to bone connections, ideal muscle length and correct size/height (for their sport/training) as well as mental attitude to want to be the best - you have someone who is going to be very good at certain sports/events. This 17/18 year old may have all these things - or he may not.
It's interesting how different people have the ability to develop strength at different rates/be more adapted to certain sports.