Obviously not a good idea. The average person doesn't even graduate from college until they are 22. But even if the age limit was changed to 18, I doubt it would really change anything. With the current age limit of 35, how many major candidates do you see who are under 40?
On a related note, I wouldn't mind seeing an upper age limit. Most people retire at 65, why shouldn't politicians? I would suggest that you have to be under 65 to run for president, or for congress, for that matter. While juvenile politicians could obviously cause problems, I sometimes wonder about some of the older ones as well.