Actually I am reading papers about large magelanic cloud, and it always mentions that getting the age using CMD (H-R diagram) is more accurate than using photometry. why CMDs give the only truly accurate ages for star clusters?CMD isn't a very common abbreviation, most people would say HR diagram.
Do you mean - how to get the age of stars from a color magnitude (ie Hertzsprung–Russell) diagram?
Yes, that's right, but that uses the photometry of each star in the cluster, not the color/magnitude of the whole cluster, because the magnitude and integrated colors for a cluster as a whole doesn't give very accurate information... Oh, wait I think i've just answered my original question!!! .... i got confused too now!Not sure I understand the question. You use photometry to get the color index of the star. Then plotted on the HR diagram gives you the age (or at least the evolutionary state of the star) for an accurate age you would also need to know metalicity and mass.