They stagnated Arnold's age for most of the series but when the show moved to ABC for the last season one of the demands Gary Coleman put in is that Arnold at least become high school aged. AS others have said, Arnold is a high school freshman in season eight making him around 14 or 15. They even at one point considered giving him a mustache to convey he'd finally hit puberty.
When there's no more room in Hollywood, remakes shall walk the Earth.

I think they tried to keep Arnold as young as possible due to Gary Colemans stunted growth

He was starting his freshman year of high school. That means he had to be either 14 or 15. This is odd, because Arnold was 8 the first season. Did Arnold fail a grade during the series? He did have Mrs. Chung for his teacher two seasons in a row, right?