It's not. Your work-ethic, organisational abilities, networks of people you know and the work you actually do is much more important than some theoretic decline - furthermore live-style and habits might have a much bigger influence than age. If you are stressed out and lack sleep you'll get dumb very fast.

I'm fairly certain that cognitive abilities do not decline across the board upon reaching 30. Ability to learn probably does, but other aspects of intelligence such as memory and emotional intelligence probably do not peak until much later in life.

I didn't start my PhD until well after 30. I don't think that it hindered me.