M.P.
My son started repeating things at about 16 or 17m. He's now 18m and can put 4 words together into phrases. The dr. told me that's very advanced. When my son was 12m he said 2 word phrases mostly.
There's a big difference from child to child though. I think it partially has to do with their brain and speech development and partially to do with their environment. My husband and I talk noooonstop. Going up stairs we count each stair together. While getting dressed, we think aloud ie. "Now We're putting on the first sock, then the second sock. Then we'll need some shoes..." so my son is constantly exposed to words. We speak in french, english, and sign language and he can communicate in all 3 languages. He must know 100 words by now; at 17m it was something like 60. But I'm told he's unusually talkative for his age. I've met 3 year olds who don't form coherent sentences and they're not developmentally delayed; it's just that nobody talks to them and they have a quiet family and are in daycare and only communicate with babies, not adults constantly talking to them.