What age should I get my child tested for allergies. She is 18 months.?

No specific age. Allergy testing can be done at any age. Some children get allergic conditions such as food allergies, asthma and atopic dermatitis at very young age such as 3 - 4 months. In such cases identifying the culprit allergen becomes important. Allergy testing can be done by skin scratch testing or through blood testing but remember skin testing is more sensitive than blood testing.
Anyage with symptoms. Although the prevailing concept has been allergy testing, both specific ige and skin tests, are not sensitive for infants and toddlers, few data is available. Positive findings might be significant to confirm observations. Negative results might not rule out specific allergies. If a toddler has significant allergic symptoms, testing might help to identify triggers and direct preventive measures.