3. if(@years_old = 4 and @Gl >= 15) and if(@years_old = 3 and @Gl >= 14) can be replaced with one condition if(@years_old >= 3 and @Gl >= 14)
These are not logically equivalent. The OP's first condition will only return records when @years_old = 4 if @GL is 15 or greater. Your combined version will return @years_old = 4 when @GL = 14.
You're correct, Doug. My bad.
Replacing it with one condition:
IF (@years_old = 4 and @Gl >= 15) OR (@years_old = 3 and @Gl >= 14)