I switched from full stack to ML at 38. I was preparing for a few years by doing courses, personal projects and following new papers as they come up. When I got hired there was no issue with the fact that I was self trained - they have been interviewing for 6 months and failing every candidate by then.
Soon after I got hired I was myself part of the interviewing committee and saw the hiring situation first hand - most candidates can't program their way out of a shoe or explain how the most commonly used neural net layers work. I mean, I had one who couldn't open a TXT file in 30 minutes, the data file he was supposed to use in the problem.