Well, actually, you can estimate, a few weeks wouldn't exactly be mature for a betta- you can tell by size mostly, color, curls, and other things. Most pet shop bettas are around 2 yrs old, while females often go as low as 4 months, but my lfs has males that are pretty young as well.It could be due to old age (you can never excatly tell if the Betta has been alive for two years, or a few weeks)