Roger Brown (1973), in his seminal work, A First Language: The Early Stages, suggested that instead of using the average number of words, syllables or age to index language development, it would be more useful to measure the Mean Length of Utterance in terms of morphemes (MLU).
Brown's stages of langage development are as follows:
Stage 1: 15-30 Months, MLU 1.75 (two-word stage after 50-60 single word utterances)
Stage 2: 28-36 Months, MLU 2.25
Stage 3: 36-42 Months, MLU 2.75
Stage 4: 40-46 Months, MLU 3.50
Stage 5: 42-53 Months, MLU 4.00
Note. Stage I begins as soon as the MLU goes above 1.0 (or multi-word utterances) and ends at 2.0, Stage II goes from 2.0 to 2.50.
For details, see the attachment.