How old are largemouth bass based on their length? And how much do they weigh? Here’s a quick guide to approximate the age and weight of your catches, based on length. In general, the age of a largemouth bass can be approximated by its length, but it's important to remember that a number of factors, including diet, habitat, and the particular genetics of the fish can influence its growth rate. Therefore, these are very rough estimates and actual ages can vary. Also, location is a big factor, so