The exponential growth has been something like 3-4%/year, or 2x in 20 years. Though it's hard to find useful statistics that take different job titles and changing nature of the industry properly into account.

If you had asked me in 2010, I would have said that the median software developer lasts 5-10 years in the industry. A lot of people left the field after the dot-com bubble burst. The same happened again in a smaller scale in the late 2000s, at least in countries where the financial crisis was a real-world event (and not just something you heard about in the news). But now there has been ~15 years of sustained growth, and the ratio of mid-career developers to juniors may even be higher than usual.