The good news: if you're in your 20s now, you have 60 or 70 years of life left, and scientific advances being what they are, they'll probably be mostly good years. That's a lot of time. Also, the common trope about poets and mathematicians peaking at 20 is the exception, not the rule, and doesn't apply to modern times anyway (mathematics is so specialized that modern mathematicians are just getting started at 25-30). The actual cognitive peak is somewhere around 50, and the drop-off after that is health-related rather than hard-coded, so if you stay healthy, even into your 80s the decline will be slight to nonexistent. Using vocabulary as an example, you probably won't gain many words from 50 to 80, but if you're healthy, you won't lose many either. So yes, if you're 25 or even 50, you have a hell of a lot of time left.
The bad news: society is incredibly ageist, and as you get older, the amount you need to have achieved in order to be taken seriously ratchets up. Do you think Mark Zuckerberg (who is as much a VC invention as most celebrities are Hollywood inventions) would have gotten the support, mentoring, and funding that he did if he were 36 instead of 20? Almost impossible. Being young is a hard-to-tap (if you're not perceived as being exceptional, you'll be typecast as a "young pup" and not taken seriously) but extremely powerful asset in business: the "boy wonder" archetype. Perhaps it's utterly moronic that it works, but it is very powerful when it hits.
This is even worse in the big-box corporate world than in technology, where your age and job title define who you are as a person. In big-box corporate, VP at 35 means you're serious, whereas VP at 50 is a consolation prize. This is why people in that world bump their college degrees up 5 years and pay for plastic surgery, the latter being an investment that pays itself off immensely. It's not just that people want to appear younger; it's that they benefit immensely by the appearance of being in their current position at a younger age.
So, you are running out of time, not because of biology, but because of the general shallowness of most people. At age 40, you have one-fifth as many options as you did at 22. It sucks, but that's the way the world works now.