Hi .

can any one maybe tell me +- how old my female is?

Hi there, there is no accurate way of knowing the age of the tarantula by size alone.
I would take a big guess and say it is about 4th instar judging by the size and it is a Brachopelma species.
How do you manage to say it is a female at that size? :blink:

That is what you would call positive thinking, I normally call spiders "she" unless it has been sexed.

If you get hold of the breeder and ask when they molted to/from first instr that will give you a age but it could be a couple months old to years depending on the growth rate, but as a general sense the hobby does not work with age but more with size, if you do work with age then you would have more or less a guestimation age unless you hatched them yourself.

Here is an example for you, I got a 1st instr B.boehmei on Nov 05 2011 at the Auction it is now 2 years later and here is the same spider I got on the 5 Nov 2011.