A player's peak is when he's at his very best. A number of All Time great defenseman have extended primes, well into their 30s. Coffey, who's always remembered for quckly falling off when he left Detroit, still won a Norris at the age of 35. Bourque, Lidstrom, Chelios, still great at 35.
A large number of players born from 1960 to 1965 (in all of the major sports), had extended primes. What was in the water back then btw?
Chara might have been better in his 30s, than he was in his 20s.
Even people thinking that Hellybuyck is leaving his prime, I'm not so sure about that. I could see him playing well into his 40s, since his style isn't overly taxing compared to other goalies.