Would you consider a boy of age 18 with height 187cms "too tall" or just "tall"?

Depends: This depends on your definition of "too".Since my father was taller than that at 18, and I always just considered him tall, I guess it depends on your cultural perspective. For some populations this would be exceedingly tall, in others not so much.

Just Tall: Your height is at the 95th percentile on growth chart meaning you are just tall and within the normal limits for height. At age 18 yrs you are mostly done with growing & near your maximum adult height. Your weight is not given but important to know. Height is genetically determined. If you have had normal health visits with your doctor, there is no need for concern. Tallness is a nice trait too!