Generally at 18-24 months children who have delayed speech are noted. Parents may express initial concerns when comparing their child to an older sibling and their speech development or children of friends who may have developed expressive language skills at an early stage.
If by age 18 months a child is not trying to make animal or motor noises, imitate and say a few words, using mostly gestures to communicate, then it may be time to at least have them seen for a screening. The early intervention program in Illinois is a good first step in having development of a child evaluated and parents can self-refer if their concern is greater than that of their pediatrician.
Waiting till a child gets older could delay early intervention which might identify and provide help for delayed speech and language. Children who have a large gap between what they understand and what they can say often get frustrated. Better to have an early intervention professional talk with you and evaluate or observe your child.