Not talking

My 19 month old isn’t talking at all… we read books everyday, narrate what we are doing, seeing, hearing, smelling, constantly talking to him, asking questions .. everything. He will not mimic words though. The ONLY thing he says is “ack” for our cat Jack. “Mama” if he’s crying.. but that’s it. He understands/follows directions and what we are saying perfectly fine. I’m starting to worry. Does anyone have experience with late talkers? When did yours start talking?