The Gladwell book is so bad it's laughable. It's a gross misrepresentation of the work of Anders Ericsson, with completely unfounded extrapolations. So bad it prompted Ericsson to get a non-academic writer to work with him to write his own popular science version of his work, which is titled "Peak". Outliers is the worst example I know of of popular authors bending facts to make a narrative they want to tell.
In a nutshell, Ericsson never said "10k hours will make you world class" or "it takes 10k hours for mastery". He said (BIG simplifying paraphrase here) "the most notable difference we saw in elite performers between sub-elites and elites was practice, with most elites having achieved roughly 10k hours, or 10 years of 3 hours a day". He also limited this to PRACTICE, a part Gladwell totally missed. (ie the Beatles example is complete bullshit, gigs are not practice). He never said it was either necessary (lots of music prodigies are world class in way less) or sufficient (literally millions of people do 10k hours and get just decent).
So... throw that book in the garbage and make your decision on other criteria. There are many, many good books on learning and performance, but not by popular hacks like Gladwell. Anecdotally, I know lots of people who started new pursuits (and got very good), or successfully switched careers or started new businesses in their 40s.
It's really not about age, it's about all the parameters that are typically associated with age. As in, by the time people hit their 40's, most people can no longer carve out the time to consistently spend time learning a new thing, and are way out of practice at learning. Most, not all. I know lots of professional musicians, for example, who took up some other hobby late in life and got very very good, because their entire life has been constructed around leaving time for effective learning, and they are experts at learning something on their own. If you can rearrange life to enable you to learn effectively (or found a company, or whatever), 43 is a great age.