Choose a career, and stay with it for life?
Perhaps that's what many people used to do, and maybe still do -- but consider this man:
Wilhelm Herschel, discoverer of the planet URANUS -- born 18 years before Mozart, and one of 6 children of a bandsman in the royal footguards in Hanover, Germany.
All those siblings learned musical instruments from very young ages, Wilhelm's being both violin and oboe. At about 21 he and one of his brothers emigrated to England, where Wilhelm eked out a living as a music copyist. He writes, [Ref.1.]
...so numerous a family would not permit my father, with his scanty circumstances to bestow much on the education of his children... I loved music to an excess, and made a considerable progress in it... The difficulty of succeeding in London induced me to visit some places in the country, and after some years spent in Newcastle, Leeds etc, I was chosen organist at Halifax in Yorkshire.
In this rather lonely period in his 20s, Herschel composed symphonies, concertos and sonatas. He also gave many performances and was even invited to lead an Edinburgh orchestra in the performance of some of his own works.
(In the background you can hear some of his Oboe Concerto in C Major.)
At 28 he was appointed Organist and Choirmaster of the Octagon Chapel in Bath (an ancient Roman city near Bristol), a position he held for about 15 years. He wrote:
My situation proved a very profitable one, as I soon fell into all the public business of the Concerts, the Rooms etc... and many times after a fatiguind day of 14 or 16 hours I retired at night with the greatest avidity to unbend the mind, with a few propositions in Maclaurin's (calculus of) Fluxions or other books of that sort...
From his job, and his performing, he was soon able to invite some of his siblings over from Hanover, particularly his younger sister Caroline.
By the time Herschel was 40 he had a reputation as an oboist, teacher, and composer (though not of course a Mozart, who was by then 22.) Yet, by the time Herschel was 43 he would suddenly acquire an even greater reputation. He wrote: [Ref.1]
Among other mathematical subjects, optics and astronomy came in turn, and when I read of the many charming discoveries that had been made by means of the telescope, I was so delighted that I wished to see the heavens with my own eyes... the price quoted for a telescope seemed to me so extravagant that I formed the resolution to make one myself... I persisted for some years... till to my infinite satisfaction I saw Saturn in the year 1774 through a 5 foot Newtonian nreflector of my own making... I used frequently to run from the harpsichord at the theater to look at thye stars during the time of an act and return to the next Music...
As we said, at 43 he made that major discovery, the large and featureless 7th planet, Uranus. King George III asked to see him and his telescope, and offered to support him as an astronomer, if he would come permanently to London. Thus ended his first 4 decades of music, and began his 2nd 4 decades (of astronomy.)
At the age of 50 Herschel married, and one son was born, who later also became a well-known astronomer.
Wilhelm Herschel discovered satellites of both Saturn and Uranus, hundreds of binary stars, and thousands of nebulae.
Near the end of his life he was knighted, and elected the first President of the newly formed Royal Astronomical Society. He died in his sleep at the age of 84, having enjoyed 2 careers, and there is a memorial stone to the German musician on the floor of Westminster Abbey.
References:
1. From Program Notes, Davis Jerome; provided with the CD from The Mozart Orchestra, conducted by Davis Jerome, Newport Classics, RI, 02909.
2. "The Age of Wonder", by Richard Holmes; Vintage Books, NY (2010).
3. "Sir William Herschel, his life and works", E.S.Holden (1881); Charles Scribners & Son, NY.