What are the signs of mental retardation at age 16 months? My sons says 7 words, and doesnt make sentences yet

Language development. Varies a great deal. Not all toddlers develop language at the same rate or in the same way. By 2 years of age, children typically begin to connect words, such as “go” and “bye, bye” to make the simple sentence “Go bye-bye.” By 3 years, many toddlers are able to form a variety of sentences with three or four words. Developmental testing by a trained professional can help identifies delays.