If they've achieved success, who is to say they're past their prime? Prime in what regard? How would you even begin to define "well past their prime"? Your premise contains an immense contradiction: if they've achieved success, they're probably not well past their prime.
Wisdom is far more important than sheer mental speed and uptake, and the proof of that is in the fact that the 20 year old Silicon Valley zillionaire is mostly a myth.
Most famous, successful tech entrepreneurs were closer to 30 or 35 when they got started or hit their stride, than 20.
Based on a lot of historical examples, prime for a tech entrepreneur, is more likely between 10 and 20 years past the age of 20. Stop buying into the bullshit hype about 20 year old entrepreneurs, it's mostly a fraud.
Here's a simple example list (not all from SV, because that's entirely irrelevant):
Paul Graham (31, Viaweb), Jan Koum (33, WhatsApp), Brian Acton (37, WhatsApp), Ev Williams (34, Twitter), Jack Dorsey (33, Square), Elon Musk (32, Tesla), Garrett Camp (30, Uber), Travis Kalanick (32, Uber), Reed Hastings (37), Eric Lefkofsky (39, Groupon), Andrew Mason (29), Reid Hoffman (36, LinkedIn), Jack Ma (35, Alibaba), Jeff Bezos (30), Jerry Sanders (33), Marc Benioff (35), Peter Norton (39), Larry Ellison (33), Mitch Kapor (32), Leonard Bosack (32), Sandy Lerner (29), Gordon Moore (39), Mark Cuban (37), Scott Cook (31), Nolan Bushnell (29), Irwin Jacobs (52), David Duffield (46, PeopleSoft), Thomas Siebel (41, Siebel Systems), John McAfee (42), Gary Hendrix (32, Symantec), Scott McNealy (28), Pierre Omidyar (28), Jim Clark (38 for SGI, and 49 for Netscape), Charles Wang (32, CA), David Packard (27), John Warnock (42, Adobe), Robert Noyce (30 at Fairchild, 41 for Intel), Rod Canion (37, Compaq), Jen-Hsun Huang (30, nVidia), Charles Brewer (36, Mindspring), William Shockley (45), John Walker (32, Autodesk), Halsey Minor (30, CNet), Ashar Aziz (44, FireEye), Kevin O'Connor (36, DoubleClick), Steve Kirsch (38, Infoseek), Michael McNeilly (28, Applied Materials), Richard Egan (43, EMC), Hasso Plattner (28, SAP), Pradeep Sindhu (43, Juniper), Peter Thiel (37, Palantir), Jay Walker (42, priceline.com), Pony Ma (27, Tencent), Robin Li (32, Baidu), Liu Qiangdong (29, JD.com), Lei Jun (40, Xiaomi), Ren Zhengfei (38, Huawei), Arkady Volozh (36, Yandex), Hiroshi Mikitani (34, Rakuten), Morris Chang (56, Taiwan Semi)
And of course Steve Jobs, who I'd say was still in his prime during the last decade of his life, when he was in his 50's (he clearly did his best work at Apple during the iPod / iPhone years). He was 31 when he bought The Graphics Group, and he basically created an entirely new company that became Pixar.
It's not a coincidence it holds true from Silicon Valley all the way to China. You can see from the list above, the super successful 20 year old founder myth could hardly be further from reality, it is the extreme exception. It's a shame more young entrepreneurs don't know this.