My own attempt at becoming an astronomer might prove enlightening!
I knew that Mathematics and Physics were important subjects so studied hard these two subjects at Secondary school where I eventually gained 8 O levels and 3 A levels.
My grades at A level in 1975 were such that I gained entry into University College London to study B.Sc. Astronomy (the entry requirements were C in Physics and a D in Mathematics). I sat 3 A levels and gained grades: A Physics, C SMP Mathematics and D in Chemistry. In those days only the top 10% went to university.
Nowadays it is more than 40% and the number of grade A’s has risen from 2% to 25% in Physics hence offers are now AAA or A* A A.
After 3 long years and much sport I was awarded a Desmond (2nd Class division II) honours degree which normally meant the end of the line for an aspiring astronomer. Luckily after 2 years in the civil service and 1 year at Durham University gaining a Teacher qualification, I applied for and was accepted on an M.Sc. degree at the University of Edinburgh. The MSc gained in 1982 enabled me to apply to 6 universities to study for a Ph.D. I was given offers by Durham, Birmingham, Imperial College, Bristol (but not UCL or Edinburgh ha ha) and eventually chose Durham University. I was very happy at Durham University (1982 - 1987). It has an excellent Astronomy department.
There were at least 5 or 6 PhD students studying astrophysics each year at Durham and perhaps 100 every year in the United Kingdom but most move on after gaining their doctorate to work in Industry as a PhD is highly prized. I went into Teaching!
From a pool of 500 would-be undergraduates who wish to study Astronomy perhaps 450 gain a degree, 200 go on to study a higher degree, 100 try for a PhD and 50 gain a PhD and then succeed in gaining a postdoctorate research position lasting 2 years. Very few gain a position as a lecturer or do research as a professional astronomer - 10 to 15 perhaps? So if you do study Physics or Astrophysics only 2% manage to eventually become a professional astronomer. Be warned.
I will research the exact numbers but it must be 500 start at the age of 18 and 10 succeed by the time they reach 27. A salutary note. Plus I gained my degrees when Fees and Grants were awarded and thus had no loans to pay off. Nowadays you would be put off by the enormous debt burden I imagine. I became a Fellow of the Royal Astronomical Society in 1984 FRAS.