Generally we would expect a bat to have wide grain if it has less than 6 grains on the face. The width of the grain is entirely dependent upon how fast the tree has grown, each grain represents one years growth. The factors that effect the rate of growth are the soil quality and amount of water available.
Generally 12 to 18 years old tree gives a wider grain and one with 25 years or more a narrower grain. A narrow grain bat will certainly play well, quicker, but will not have a particularly long life.
On the other hand a wider grain bat (with as little as 4 grains on the face) will play as well, given time, as a narrow grain, it will also , without doubt have a longer life span. The reason for this is that the wood is not as old, so it is stronger and will stand up better to the beating with some of the very hard, cheap balls used in matches these days.