Ok but I think he was asking, like when you go into a pet store, how can you gage a betta's age?? My Zed is a lot Smaller than My W, and I bought Z because he was so tiny. I mean tiny. I bought my simi(who passed away this morning) at a walmart because I felt so sorry for him. I think he was really old when I got him because he was very faded in colour and was very big. I thought maybe someone was weeding out thier old breeding stock or something. I'm not sure. But anywho, how can you tell an age of a betta?
They are usually about 3-6 months old at the stores. As far as I know, the Bettas that we buy at the store are either 'culls' from a breeding pair or are the offspring from breeding intended just to go to the stores. They usually aren't very old since the fish farms want to maximize profit and ship them out as soon as they can. I'm pretty sure our double-combtail Mickey, was 3 months or maybe less when we got him since he was so tiny and has grown quite a bit since then. We got Teddy Bear, our feathered betta with a bad swim bladder, from his breeder at a Betta Show when he was 8 months old and he was about 3" long then.
It's hard to gauge their age as they can vary quite a bit in size and with different personalities, even their activity level isn't always an accurate measure. Generally as they get older, they do rest more and eat less, but that doesn't help determine store age.
I read your other post first - welcome to FishLore! We've learned so much since we started participating in the forum and you'll find plenty of us Betta parents here. I know how you felt when you bought Simi.. we have the hardest time not picking up the worst-off bettas at the store. We were going to start rescuing the sick ones and then find them good homes but it didn't take long for us to see that good home would be here with us. Unfortunately, it sounds like Simi was most likely only 5-6 months but the victim of damage from water toxins at the store. :'(