IBM said it has taken down a job posting that raised questions about age discrimination about six weeks after it laid off 419 employees at its Essex Junction plant. The advertisement was for entry-level development engineers in Essex Junction and in Albany and Hopewell Junction, N.Y. and required that ‘‘all applicants must have graduation dates of January 2010 or later.’’ IBM Spokesman Doug Shelton said Friday that the posting would be removed and replaced. ‘‘It was a thoughtless mistake,’’ he said. An employee group called Alliance@IBM alerted media outlets to the ad. The civil rights unit of the Vermont attorney general’s office also was made aware of the ad. ‘‘You cannot discriminate on the basis of one’s age, that’s just clear,’’ Attorney General Bill Sorrell said.