skilled at', is wrong usage of preposition ,'at..skilled in ',is the right preposition usage and refers to acquisition of specific skills in a field of endeavour.Good at is similarly wrong ,good in is the right usage it descibes the level of proficiency of the skill under consideration. He is good in mathematics subject or the science subjects and not good at maths.He is skilled in surgery and not skilled at surgery.He acquired his surgical skills at an early age of 10.
