Unfortunately as far as I can tell and know, once they hit a certain age it's very difficult to tell their age until they get into "old age" around 2-3 years, sometimes lucky Betta's can get to 5 years! In which case they can get to be about 2 inches just in body size, generally their color will fade a little more whey they hit old age.
And yeah, bubble nests are not a good way to show age as many of my young mature Betta's don't even make them. I've actually only had three Betta's out of all, alive and dead, that have made nests.
That's just what I know, I'm sure other's know plenty more though about this topic, probably some breeders ;-D
And lol to this quote! "We are but padawan's to our Jedi Betta Knights!"
Originally Posted by Juditko
This is a great question...never seen it discussed here yet. Signing up for the thread so I get to see what our Jedi members think...