It does seem that people who are career successful are successful early in life. I’m not sure if this is a credit to nature or if as one ages their energies are diverted elsewhere (e.g. being a successful parent).
Then again, perhaps we only read about the young successful and their IPOs (not that success needs to result in a need for investors or selling your business to shareholders, but it is a visible metric) as it is considered more newsworthy.
I think one thing that happens is that the ‘hopefulness’ of youth becomes replaced by the anxiety that one is running out of time. Running out of time to enjoy and be happy in life (and therefore less willing to put everything into your job). Running out of time to make those changes in direction and take the risky moves that are part of great success. Etc. Add to this that you feel (for good reason) that you are looked upon differently by others as you age, one has less of that ‘nothing is going to stop me’ optimism that drives you. I think it's important to bring carry-over early successes into later years in life. That provides a bedrock that keeps the ‘hopefulness’ alive - and keeps you ambitious. That doesn't mean you can't start late and do great things. It's just harder.