Your best earning power in knowledge based industries is in your fifties, though we are edging towards sixties. Other industries rely on other capital such as farming (land), sports persons (fitness) and modelling (youthful looks).
Having accumulated knowledge and experience in my field, some 25 years of it, I seem to now be reaping the benefit. I can get £6,000 for a days work though spend around 4 days a month topping my skills up which is hard mental work. Some days I do work, invoice, forget about it and customers still pay me. I am bad at my own paperwork, but good at theirs.
My best year was around £600,000 net (2016) of tax and my worst around £6,000 net of tax (1988). I was young then. This month according to my bank I have received from customers £7,356 plus £1,230, plus £4,110 plus £23.49 (not sure what that was!) plus £1,530, plus £8,400 - all figures include the 20% VAT I have to account to Customs for.
I pay a lot of tax, give a lot of money away and try not to worry about the future.