In part 4 of Rand's "The Fountainhead," Peter's career is in decline. He is desperate for a big project. He decides he wants to tackle the design for Cortlandt Homes, a project for low-income housing. He stays up all night and draws sketches of the project. He realizes he can't do it so he calls Roark. After Keating and Roark make a deal for Roark to design Cortlandt Homes, and Keating to take the credit for it Peter decides on his way out the door to share the sketches he has drawn with Roark. He tells Roark as he hands him the sketches of the buildings he has drawn that he had not shown them to anyone else.
"Peter says, " I just want you to tell me if there's any..."
He handed to Roark six of his canvases. Roark looked at them, one after another. He took longer time than he needed. When he could trust himself to lift his eyes, he shook his head in silent answer to the word Keating had not pronounced. pgs.599-609
"It's too late, Peter," he said gently. Keating nodded. "Guess I...knew that.'" (pg, 608)
Roark means it is too late for Peter to try and become a good architech. He bowed to public opinion so long that Peter just can't design and the sketches of the Cortlandt Homes that Peter drew, and that he is now showing to Roark just are not good.
We are to assume that Peter wanted originally to be a painter and his mom drove him towards architecture. He always hated architecture.
When you start painting, you need to learn. As you paint, you progress. The painting Keating showed Roark was a beginner's painting, perhaps showing promise, but for Keating to become a true artist and painter, he would have needed a lifetime of painting. Now he was too old to really have time to accomplish becoming an accomplished painter. So Roark said it was too late. Much like trying to learn an instrument in your adult years; difficult to become really good.