In the episode "The Heart Attack", George said he was 33, yet in "The Andrea Doria" which was made nearly six years later, he said he was 34. Kramer once said that Jerry was in his late thirties and since George and Jerry went to school together, George should be older than 34.
Actually, George mentions his age in "The Heart Attack," and since he and Jerry are four months apart (according to "The Butter Shave") we can guess their ages from that episode. I'm not sure, but I think George said he was thirty-two. Since that was a season two episode, it's been 7 years, which means Jerry and George are probably thirty-nine.
Although Jerry looks much older if you compare him to early episodes, he still looks great for his age--he doesn't look like he's in his forties at all. Michael Richards, however, has definitely aged, especially in the past two years or so. Although he could pass for younger, he does look 47.