My dad was a pretty smart guy. He didn't talk until he was two. When he did start talking it was in complete sentences. I guess he wanted to do it right the first time!
My son said only a few words for the longest time, but I knew he was smart by his ability to pull compplex items apart and put them back together again. Then, out of nowhere, here comes sentences.
I should remind my sister of that when she frets that her 18 month old daughter hardly speaks.