My 25 weeker at 17.5mths (14mths corrected) is saying mumumum, dadadada, ahha (so cute) but is not really saying any discernible words more just babble at this time.
My 28 weeker is 2.5yrs and at least 12 mths behind in her speech and is seeing a speechie.
My 35 weeker was a real babbler from 6mths and now at 3.5yrs could talk the leg of a chair.
But you have to remember prem or not each child is different and does things at their own pace. All 7 of mine have done things different.
She is only 5mths developmentally so she is still very young...I wouldn't be worrying yet.