Unfortunately, Bruffin is right. Allergies can develop at any time. My sister has developed environmental and animal allergies in her mid twenties 
My ds recently tested positive for cashew nuts through skin prick testing. It's a really scary time when you discover that you have an allergic child, and reading through a lot of the threads on here - as well as starting a few of my own! - helped me to be prepared for the testing process and to have some questions prepared rather than going into the whole thing blind.
From my experience, this is what the testing process involves: the tester will put small drops of liquid, each containing different allergens, onto your dd's forearm and prick through them into the skin. If there is a reaction to any of the allergens, a weal will appear on the skin and the tester will measure it. We were told that a reaction between 1 and 2mm could probably be discounted (ds had a 2mm weal to apple and he has since been eating it with no problems) but anything over that was likely to indicate that there would be a reaction. Ds is also allergic to egg and had a 3mm reaction to cooked, but 13mm to raw, so I'm hoping that in the future he'll at least be able to eat egg baked into things like cakes, etc.
Ds also had blood taken for testing. A small amount of allergen is added to the blood, and the immune response (IgE level) measured. The overall IgE level of the blood is also taken and this gives a context in which to interpret the test results. For this reason, it is not necessarily helpful to compare your own test results with other peoples as each individual's scores need to be interpreted within the context of their own overall IgE level. (Does that make sense?!)
Each of these methods of testing will only show the likelihood of an allergy being present. They can not predict how severe a reaction could be - there are many other outside factors that can influence that, and what might be a mild reaction one day could become a severe one on another. It is useful to have the two sets of tests to compare - in our case, we were told that DS was allergic to wheat as his IgE level was slightly raised on the blood tests, but when the skin prick was negative, we tried him with wheat and he is now happily munching on toast at breakfast time!
A food challenge will only be undertaken if the patient is showing progressively lower scores/weals to an allergen. A small amount of the food in question will be rubbed onto the skin, then the lips, then the tongue and so on at intervals of ten minutes or more and the test will progress for as long as no reaction is shown. A challenge is not recommended in anything other than a clinical setting, as obviously if there is an adverse reaction to the food the consequences could be very serious indeed. Food challenges are regarded as the only real way to show if an allergy is still present, as with both skin-prick and blood testing it is possible to have false positive, and false negative, results.
Anyone who has ever read one of my threads will know that I am incapable of writing a short post 
. I hope this information is useful to you, and helps you feel a little more prepared for what is to come. In the meantime, you are right to avoid nuts and nut-containing food.You might find this current thread useful for a discussion on whether or not to avoid 'may contain' foods. I have been following it with interest, and it has opened my eyes a little bit to the way I look at food labels.
I hope the information I have given here is accurate, I am sure someone will correct me if I have got anything wrong! Good luck 