This is an extremely common problem. What you are describing is called allergic contact dermatitis in medical terms. It is an itchy red rash that forms when your skin comes in contact with a substance (in your case, non-precious metal jewelry) which is normally harmless. The most common element that causes the sensitivity is nickel. Nickel allergy can occur at any age. Nickel is not only found in inexpensive metal jewelry but also in eyeglass frames, zippers, hooks and belt buckles so be careful with these as well. Treatments can help reduce the symptoms of the dermatitis but you are correct in that once you develop an allergy to nickel you will always be sensitive to it and therefore the easiest, most effective, treatment is simply to avoid contact with anything containing nickel. If you inadvertently come into contact with a nickel-containing substance and develop a more severe rash that does not respond to simple removal of the substance and over the counter anti-itch creams it is important to see a dermatologist
or your primary care doctor
who can prescribe topical (or sometimes oral) corticosteroids to help your symptoms. Do not scratch the rash as this can lead to secondary bacterial infections.