Sexing/ageing

Ok so i've always like catfish in the sence i'd buy a bn because it was cute and stick it in a tank on it's own.
But i always assumed size meant age...i recently discovered this is not the case with catfish and bn especially, so my question is.....if i buy a longfin how do i know roughly how old it is? and if it's by size do i measure just the body or the tail aswell?
If not how can i tell ? is there a certain feature such as the bristles between the eyes that only begin to grow once they reach a certain age or something else?
Or is it just hit and miss with them?

I have been looking into this too...
When I went to get my longfins I was told they were 2" but didnt realise she ment the tail aswell so they were tiny in the body. Smaller than I like to buy them at...

Longfins are best measured on body size, the length from the snout to the start of the tail.

On average a common BN will reach breeding size (5-6cms) at about 10 months, but longfins are slower growing so would probably reach maturity later. As you say growth rate is affected by so many things you could never know the age of a fish just by looking at it, even fish kept under optimum conditions will grow at different rates.

If your looking for BN's reaching breeding size then your looking for bristles starting to grow on the males and females will starting to get gravid.