It is hard to tell a bettas age just by looking at them. Generally, if they are smaller than the average, you can assume they are not fully mature yet, and being inactive can be a sign of old age.
Okay thats good info. I have had my oldest for 7 months now, so I am guessing then he is at least a year old then. He is still very active and appears healthy. I just hope I can keep it that way :)