He's 25 years old now. I know that most d-men mature slower than forwards, but if you're not an NHLer at 25, the clock becomes a big factor.
Poolman is a late bloomer, the conventional wisdom shouldn't be applied to his trajectory. When most guys were already sophomores in college he was still playing in the USHL. He developed late, but everywhere he's played he's gotten better and better as he got more comfortable. He's going to be alright.