In the talk yourself horse bit, he said something about if a person wasn't jumping big tracks by the age of 27/28 that they'd missed the boat. I presume he means that they never will.
It depends what he means by big classes (don't by H&H unless there are big show reports/previews. Some people might bring on young horses before they are 28, and wait for a horse that they think is the "one" to keep and bring up to a high level.
I think he was spot on with his comments tbh. It's very rare (and indeed very hard) for people to get to grips with the biggest tracks later in life if they haven't had the experience when younger. Your bravery and general ideas on self-preservation tend to get much stronger as you get older