You can't exactly tell their age, the bettas in stores are usually 6 months-1 year.
All the domestic bettas in stores are the same species; betta splendens. If the betta on the site is a splenden, then the age ratio will be the same.
It doesn't matter what tail type your betta is, they're all the same through the species. :D
Over hundreds of bettas die in their small, dirty cups each day. Only you can help them live. Research and then save them, today!
Yes any tail type. And measuring length of bettas doesn't usually include their tail itself, so no problem. One thing that's helpful with that growth scale is that he gives a 1" marker so it's a bit easier to visualize. Have fun with your little guy ;)
The more I think I know, the more I think I don't know much at all!